cex: display derivations as trees

Sometimes, understanding the derivations is difficult, because they are serialized to fit in one line. For instance, the example taken from the NEWS file: %token ID %% s: a ID a: expr expr: expr ID ',' | "expr" gave First example expr • ID ',' ID $end Shift derivation $accept → [ s → [ a → [ expr → [ expr • ID ',' ] ] ID ] $end ] Second example expr • ID $end Reduce derivation $accept → [ s → [ a → [ expr • ] ID ] $end ] Printing as trees, it gives: First example expr • ID ',' ID $end Shift derivation $accept ↳ s $end ↳ a ID ↳ expr ↳ expr • ID ',' Second example expr • ID $end Reduce derivation $accept ↳ s $end ↳ a ID ↳ expr • * src/glyphs.h, src/glyphs.c (down_arrow, empty, derivation_separator): New. * src/derivation.c (derivation_print, derivation_print_impl): Rename as... (derivation_print_flat, derivation_print_flat_impl): These. (fputs_if, derivation_depth, derivation_width, derivation_print_tree) (derivation_print_tree_impl, derivation_print): New. * src/counterexample.c (print_counterexample): Adjust. * tests/conflicts.at, tests/counterexample.at, tests/diagnostics.at, * tests/report.at: Adjust.
2026-03-09 04:13:03 +00:00 · 2020-07-14 08:16:16 +02:00
parent 5544615a59
commit fff17fe8fe
9 changed files with 986 additions and 72 deletions
--- a/27
+++ b/27
@@ -46,9 +46,17 @@ Changes in the display of counterexamples.
  strings in the grammar which can be parsed in two ways due to the
  conflict.  For example:

-    Example              exp '+' exp • '/' exp
-    Shift derivation     exp → [ exp '+' exp → [ exp • '/' exp ] ]
-    Reduce derivation    exp → [ exp → [ exp '+' exp • ] '/' exp ]
+    Shift/reduce conflict on token "/":
+      Example              exp "+" exp • "/" exp
+      Shift derivation
+        exp
+        ↳ exp "+" exp
+                  ↳ exp • "/" exp
+      Example              exp "+" exp • "/" exp
+      Reduce derivation
+        exp
+        ↳ exp             "/" exp
+          ↳ exp "+" exp •

  When Bison is installed with text styling enabled, the example is actually
  shown twice, with colors highlighting the ambiguity.
@@ -59,9 +67,18 @@ Changes in the display of counterexamples.
  generates two examples that are the same up until the dot:

    First example        expr • ID ',' ID $end
-    Shift derivation     $accept → [ s → [ a → [ expr → [ expr • ID ',' ] ] ID ] $end ]
+    Shift derivation
+      $accept
+      ↳ s                      $end
+        ↳ a                 ID
+          ↳ expr
+            ↳ expr • ID ','
    Second example       expr • ID $end
-    Reduce derivation    $accept → [ s → [ a → [ expr • ] ID ] $end ]
+    Reduce derivation
+      $accept
+      ↳ s             $end
+        ↳ a        ID
+          ↳ expr •

  In these cases, the parser usually doesn't have enough lookahead to
  differentiate the two given examples.
--- a/src/counterexample.c
+++ b/src/counterexample.c
@@ -115,10 +115,11 @@ free_counterexample (counterexample *cex)
 static void
 print_counterexample (const counterexample *cex, FILE *out, const char *prefix)
 {
+  const bool flat = getenv ("YYFLAT");
  fprintf (out, "  %s%-20s ",
           prefix, cex->unifying ? _("Example") : _("First example"));
  derivation_print_leaves (cex->d1, out, prefix);
-  fprintf (out, "  %s%-20s ",
+  fprintf (out, flat ? "  %s%-20s " : "  %s%s",
           prefix, cex->shift_reduce ? _("Shift derivation") : _("First derivation"));
  derivation_print (cex->d1, out, prefix);

@@ -131,7 +132,7 @@ print_counterexample (const counterexample *cex, FILE *out, const char *prefix)
               prefix, cex->unifying ? _("Example") : _("Second example"));
      derivation_print_leaves (cex->d2, out, prefix);
    }
-  fprintf (out, "  %s%-20s ",
+  fprintf (out, flat ? "  %s%-20s " : "  %s%s",
           prefix, cex->shift_reduce ? _("Reduce derivation") : _("Second derivation"));
  derivation_print (cex->d2, out, prefix);

--- a/src/derivation.c
+++ b/src/derivation.c
@@ -22,7 +22,9 @@
 #include "derivation.h"
 #include "glyphs.h"

+#include <c-ctype.h>
 #include <gl_linked_list.h>
+#include <mbswidth.h>

 #include "system.h"
 #include "complain.h"
@@ -32,9 +34,13 @@ struct derivation
  symbol_number sym;
  derivation_list children;
  int reference_count;
+  // Color assigned for styling.  Guarantees that the derivation is
+  // always displayed with the same color, independently of the order
+  // in which the derivations are traversed.
+  int color;
 };

-static derivation d_dot = { -1, NULL, -1 };
+static derivation d_dot = { -1, NULL, -1, -1 };

 derivation *
 derivation_dot (void)
@@ -74,6 +80,7 @@ derivation_new (symbol_number sym, derivation_list children)
  deriv->sym = sym;
  deriv->children = children;
  deriv->reference_count = 0;
+  deriv->color = -1;
  return deriv;
 }

@@ -127,19 +134,235 @@ derivation_size (const derivation *deriv)
  return size;
 }

+
+static int
+max (int a, int b)
+{
+  return a < b ? b : a;
+}
+
+// Longest distance from root to leaf.
+static int
+derivation_depth (const derivation *deriv)
+{
+  if (deriv->children)
+    {
+      // Children's depth cannot be 0, even if there are no children
+      // (the case of a derivation with an empty RHS).
+      int res = 1;
+      derivation *child;
+      for (gl_list_iterator_t it = gl_list_iterator (deriv->children);
+           derivation_list_next (&it, &child);
+           )
+        res = max (res, derivation_depth (child));
+      return res + 1;
+    }
+  else
+    return 1;
+}
+
+static bool
+all_spaces (const char *s)
+{
+  while (c_isspace (*s))
+    s++;
+  return *s == '\0';
+}
+
+// Printing the derivation as trees without trailing spaces is
+// painful: we cannot simply pad one "column" before moving to the
+// next:
+//
+// exp
+// ↳ x1          e1  foo1                      x1
+//   ↳ x2        ↳ ε ↳ foo2                    ↳ x2
+//     ↳ x3            ↳ foo3                    ↳ x3
+//       ↳ "X" •         ↳ x1        foo4          ↳ "X"
+//                         ↳ x2      ↳ "quuux"
+//                           ↳ x3
+//                             ↳ "X"
+//
+// It's hard for a column to know that it's "last" to decide whether
+// to output the right-padding or not.  So when we need to pad on the
+// right to complete a column, we don't output the spaces, we
+// accumulate the width of padding in *PADDING.
+//
+// Each time we actually print something (non space), we flush that
+// padding.  When we _don't_ print something, its width is added to
+// the current padding.
+//
+// This function implements this.
+//
+// When COND is true, put S on OUT, preceeded by *PADDING white
+// spaces.  Otherwise add the width to *PADDING.  Return the width of
+// S.
+static int
+fputs_if (bool cond, FILE *out, int *padding, const char *s)
+{
+  int res = mbswidth (s, 0);
+  if (cond && !all_spaces (s))
+    {
+      fprintf (out, "%*s%s", *padding, "", s);
+      *padding = 0;
+    }
+  else
+    {
+      *padding += res;
+    }
+  return res;
+}
+
+// The width taken to report this derivation recursively down to its
+// leaves.
+static int
+derivation_width (const derivation *deriv)
+{
+  if (deriv->children)
+    {
+      const symbol *sym = symbols[deriv->sym];
+      int self_width = mbswidth (sym->tag, 0);
+
+      // Arrow and space.
+      int children_width = down_arrow_width;
+      if (gl_list_size (deriv->children) == 0)
+        // Empty rhs.
+        children_width += empty_width;
+      else
+        {
+          derivation *child;
+          for (gl_list_iterator_t it = gl_list_iterator (deriv->children);
+               derivation_list_next (&it, &child);
+               )
+            children_width
+              += derivation_separator_width + derivation_width (child);
+          // No separator at the beginning.
+          children_width -= derivation_separator_width;
+        }
+      return max (self_width, children_width);
+    }
+  else if (deriv == &d_dot)
+    {
+      return dot_width;
+    }
+  else // leaf.
+    {
+      const symbol *sym = symbols[deriv->sym];
+      return mbswidth (sym->tag, 0);
+    }
+}
+
+
+// Print DERIV for DEPTH.
+//
+// The tree is printed from top to bottom with DEPTH ranging from 0 to
+// the total depth of the tree.  DERIV should only printed when we
+// reach its depth, i.e., then DEPTH is 0.
+//
+// When DEPTH is 1 and we're on a subderivation, then we print the RHS
+// of the derivation (in DEPTH 0 we printed its LHS).
+//
+// Return the "logical printed" width.  We might have not have reached
+// that width, in which case the missing spaces are in *PADDING.
+static int
+derivation_print_tree_impl (const derivation *deriv, FILE *out,
+                            int depth, int *padding)
+{
+  const int width = derivation_width (deriv);
+
+  int res = 0;
+  if (deriv->children)
+    {
+      const symbol *sym = symbols[deriv->sym];
+      char style[20];
+      snprintf (style, 20, "cex-%d", deriv->color);
+
+      if (depth == 0 || depth == 1)
+        {
+          begin_use_class (style, out);
+          begin_use_class ("cex-step", out);
+        }
+      if (depth == 0)
+        {
+          res += fputs_if (true, out, padding, sym->tag);
+        }
+      else
+        {
+          res += fputs_if (depth == 1, out, padding, down_arrow);
+          if (gl_list_size (deriv->children) == 0)
+            // Empty rhs.
+            res += fputs_if (depth == 1, out, padding, empty);
+          else
+            {
+              bool first = true;
+              derivation *child;
+              for (gl_list_iterator_t it = gl_list_iterator (deriv->children);
+                   derivation_list_next (&it, &child);
+                   )
+                {
+                  if (!first)
+                    res += fputs_if (depth == 1, out, padding, derivation_separator);
+                  res += derivation_print_tree_impl (child, out, depth - 1, padding);
+                  first = false;
+                }
+            }
+        }
+      if (depth == 0 || depth == 1)
+        {
+          end_use_class ("cex-step", out);
+          end_use_class (style, out);
+        }
+      *padding += width - res;
+      res = width;
+    }
+  else if (deriv == &d_dot)
+    {
+      if (depth == 0)
+        begin_use_class ("cex-dot", out);
+      res += fputs_if (depth == 0, out, padding, dot);
+      if (depth == 0)
+        end_use_class ("cex-dot", out);
+    }
+  else // leaf.
+    {
+      const symbol *sym = symbols[deriv->sym];
+      if (depth == 0)
+        begin_use_class ("cex-leaf", out);
+      res += fputs_if (depth == 0, out, padding, sym->tag);
+      if (depth == 0)
+        end_use_class ("cex-leaf", out);
+    }
+  return res;
+}
+
+static void
+derivation_print_tree (const derivation *deriv, FILE *out, const char *prefix)
+{
+  fputc ('\n', out);
+  for (int depth = 0, max_depth = derivation_depth (deriv);
+       depth < max_depth; ++depth)
+    {
+      int padding = 0;
+      fprintf (out, "    %s", prefix);
+      derivation_print_tree_impl (deriv, out, depth, &padding);
+      fputc ('\n', out);
+    }
+}
+
+
 /* Print DERIV, colored according to COUNTER.
   Return false if nothing is printed.  */
 static bool
-derivation_print_impl (const derivation *deriv, FILE *out,
+derivation_print_flat_impl (derivation *deriv, FILE *out,
                            bool leaves_only,
                            int *counter, const char *prefix)
 {
  if (deriv->children)
    {
      const symbol *sym = symbols[deriv->sym];
-      char style[20];
-      snprintf (style, 20, "cex-%d", *counter);
+      deriv->color = *counter;
      ++*counter;
+      char style[20];
+      snprintf (style, 20, "cex-%d", deriv->color);
      begin_use_class (style, out);

      if (!leaves_only)
@@ -156,7 +379,8 @@ derivation_print_impl (const derivation *deriv, FILE *out,
           derivation_list_next (&it, &child);
           )
        {
-          if (derivation_print_impl (child, out, leaves_only, counter, prefix))
+          if (derivation_print_flat_impl (child, out,
+                                          leaves_only, counter, prefix))
            {
              prefix = " ";
              res = true;
@@ -194,21 +418,29 @@ derivation_print_impl (const derivation *deriv, FILE *out,
  return true;
 }

-void
-derivation_print (const derivation *deriv, FILE *out, const char *prefix)
+static void
+derivation_print_flat (const derivation *deriv, FILE *out, const char *prefix)
 {
  int counter = 0;
  fputs (prefix, out);
-  derivation_print_impl (deriv, out, false, &counter, "");
+  derivation_print_flat_impl ((derivation *)deriv, out, false, &counter, "");
  fputc ('\n', out);
 }

-
 void
 derivation_print_leaves (const derivation *deriv, FILE *out, const char *prefix)
 {
  int counter = 0;
  fputs (prefix, out);
-  derivation_print_impl (deriv, out, true, &counter, "");
+  derivation_print_flat_impl ((derivation *)deriv, out, true, &counter, "");
  fputc ('\n', out);
 }
+
+void
+derivation_print (const derivation *deriv, FILE *out, const char *prefix)
+{
+  if (getenv ("YYFLAT"))
+    derivation_print_flat (deriv, out, prefix);
+  else
+    derivation_print_tree (deriv, out, prefix);
+}
--- a/src/glyphs.c
+++ b/src/glyphs.c
@@ -36,10 +36,21 @@ static glyph_buffer_t arrow_buf;
 const char *arrow;
 int arrow_width;

+static glyph_buffer_t down_arrow_buf;
+const char *down_arrow;
+int down_arrow_width;
+
 static glyph_buffer_t dot_buf;
 const char *dot;
 int dot_width;

+static glyph_buffer_t empty_buf;
+const char *empty;
+int empty_width;
+
+const char *derivation_separator = " ";
+int derivation_separator_width = 1;
+
 typedef struct
 {
  const char **glyph;
@@ -54,7 +65,6 @@ on_success (const char *buf, size_t buflen, void *callback_arg)
  callback_arg_t *arg = (callback_arg_t *) callback_arg;
  assert (buflen < sizeof arg->buf);
  strncpy (arg->buf, buf, buflen);
-  *arg->glyph = arg->buf;
  return 1;
 }

@@ -63,7 +73,8 @@ on_failure (unsigned code MAYBE_UNUSED, const char *msg MAYBE_UNUSED,
            void *callback_arg)
 {
  callback_arg_t *arg = (callback_arg_t *) callback_arg;
-  *arg->glyph = arg->fallback;
+  assert (strlen (arg->fallback) < sizeof arg->buf);
+  strcpy (arg->buf, arg->fallback);
  return 0;
 }

@@ -74,6 +85,7 @@ glyph_set (const char **glyph,
 {
  callback_arg_t arg = { glyph, buf, fallback };
  int res = unicode_to_mb (code, on_success, on_failure, &arg);
+  *glyph = buf;
  *width = mbswidth (*glyph, 0);
  return res;
 }
@@ -83,4 +95,9 @@ glyphs_init (void)
 {
  glyph_set (&arrow,      arrow_buf,      &arrow_width,      0x2192, "->");
  glyph_set (&dot,        dot_buf,        &dot_width,        0x2022, ".");
+  glyph_set (&down_arrow, down_arrow_buf, &down_arrow_width, 0x21b3, "`->");
+  glyph_set (&empty,      empty_buf,      &empty_width,      0x03b5, "%empty");
+
+  strncat (down_arrow_buf, " ", sizeof down_arrow_buf - strlen (down_arrow_buf) - 1);
+  down_arrow_width += 1;
 }
--- a/src/glyphs.h
+++ b/src/glyphs.h
@@ -31,4 +31,16 @@ extern int arrow_width;
 extern const char *dot;
 extern int dot_width;

+/* "↳ ", below an lhs to announce the rhs.  */
+extern const char *down_arrow;
+extern int down_arrow_width;
+
+/* "ε", an empty rhs.  */
+extern const char *empty;
+extern int empty_width;
+
+/* " ", separate symbols in the rhs of a derivation.  */
+extern const char *derivation_separator;
+extern int derivation_separator_width;
+
 #endif /* GLYPHS_H */
--- a/tests/conflicts.at
+++ b/tests/conflicts.at
@@ -865,8 +865,14 @@ State 5
        1 exp: exp OP exp .
        1 exp: exp . OP exp
      Example                  exp OP exp . OP exp
-      Shift derivation         exp -> [ exp OP exp -> [ exp . OP exp ] ]
-      Reduce derivation        exp -> [ exp -> [ exp OP exp . ] OP exp ]
+      Shift derivation
+        exp
+        `-> exp OP exp
+                   `-> exp . OP exp
+      Reduce derivation
+        exp
+        `-> exp              OP exp
+            `-> exp OP exp .

 ]])

@@ -1119,7 +1125,7 @@ m4_popdef([AT_TEST])
 # else.

 AT_SETUP([Defaulted Conflicted Reduction])
-AT_KEYWORDS([report])
+AT_KEYWORDS([cex report])

 AT_DATA([input.y],
 [[%%
@@ -1207,8 +1213,14 @@ State 1
        3 num: '0' .
        4 id: '0' .
      Example                  '0' .
-      First derivation         exp -> [ num -> [ '0' . ] ]
-      Second derivation        exp -> [ id -> [ '0' . ] ]
+      First derivation
+        exp
+        `-> num
+            `-> '0' .
+      Second derivation
+        exp
+        `-> id
+            `-> '0' .



@@ -1579,6 +1591,8 @@ AT_CLEANUP

 AT_SETUP([[Unreachable States After Conflict Resolution]])

+AT_KEYWORDS([cex report])
+
 # If conflict resolution makes states unreachable, remove those states, report
 # rules that are then unused, and don't report conflicts in those states.  Test
 # what happens when a nonterminal becomes useless as a result of state removal
@@ -1754,17 +1768,29 @@ State 4
       10 reported_conflicts: . %empty
        8 reported_conflicts: . 'a'
      First example            resolved_conflict . 'a' 'a'
-      Shift derivation         start -> [ resolved_conflict reported_conflicts -> [ . 'a' ] 'a' ]
+      Shift derivation
+        start
+        `-> resolved_conflict reported_conflicts 'a'
+                              `-> . 'a'
      Second example           resolved_conflict . 'a'
-      Reduce derivation        start -> [ resolved_conflict reported_conflicts -> [ . ] 'a' ]
+      Reduce derivation
+        start
+        `-> resolved_conflict reported_conflicts 'a'
+                              `-> .

    Shift/reduce conflict on token 'a':
       10 reported_conflicts: . %empty
        9 reported_conflicts: . 'a'
      First example            resolved_conflict . 'a' 'a'
-      Shift derivation         start -> [ resolved_conflict reported_conflicts -> [ . 'a' ] 'a' ]
+      Shift derivation
+        start
+        `-> resolved_conflict reported_conflicts 'a'
+                              `-> . 'a'
      Second example           resolved_conflict . 'a'
-      Reduce derivation        start -> [ resolved_conflict reported_conflicts -> [ . ] 'a' ]
+      Reduce derivation
+        start
+        `-> resolved_conflict reported_conflicts 'a'
+                              `-> .



@@ -1781,8 +1807,12 @@ State 5
        8 reported_conflicts: 'a' .
        9 reported_conflicts: 'a' .
      Example                  'a' .
-      First derivation         reported_conflicts -> [ 'a' . ]
-      Second derivation        reported_conflicts -> [ 'a' . ]
+      First derivation
+        reported_conflicts
+        `-> 'a' .
+      Second derivation
+        reported_conflicts
+        `-> 'a' .



@@ -1904,6 +1934,8 @@ AT_CLEANUP

 AT_SETUP([[%nonassoc error actions for multiple reductions in a state]])

+AT_KEYWORDS([cex report])
+
 AT_DATA([[input.y]],
 [[%nonassoc 'a' 'b' 'c'
 %%
@@ -1965,8 +1997,14 @@ AT_CHECK([[cat input.output | sed -n '/^State 0$/,/^State 1$/p']], 0,
       12 empty_c2: . %empty
       13 empty_c3: . %empty
      Example                  . 'c'
-      First derivation         start -> [ empty_c2 -> [ . ] 'c' ]
-      Second derivation        start -> [ empty_c3 -> [ . ] 'c' ]
+      First derivation
+        start
+        `-> empty_c2 'c'
+            `-> .
+      Second derivation
+        start
+        `-> empty_c3 'c'
+            `-> .



--- a/tests/counterexample.at
+++ b/tests/counterexample.at
@@ -17,14 +17,23 @@

 AT_BANNER([[Counterexamples.]])

-# AT_BISON_CHECK_CEX(EXPERR)
-# --------------------------
+# AT_BISON_CHECK_CEX(TREE, FLAT)
+# ------------------------------
 m4_define([AT_BISON_CHECK_CEX],
 [AT_KEYWORDS([cex])
-AT_DATA([expout], [$1])
+
+AT_BISON_CHECK([-Wcounterexamples input.y], [0], [], [stderr])
+# FIXME: Avoid trailing white spaces.
+AT_CHECK([[sed -e 's/time limit exceeded: [0-9][.0-9]*/time limit exceeded: XXX/g;s/ *$//;' stderr]],
+         [], [$1])
+
+m4_pushdef([AT_SET_ENV_IF],
+           [[YYFLAT=1; export YYFLAT;]]m4_defn([AT_SET_ENV_IF]))
 AT_BISON_CHECK([-Wcounterexamples input.y], [0], [], [stderr])
 AT_CHECK([[sed -e 's/time limit exceeded: [0-9][.0-9]*/time limit exceeded: XXX/g' stderr]],
-         [], [expout])
+         [], [$2])
+m4_popdef([AT_SET_ENV_IF])
+
 ])

 ## --------------------- ##
@@ -45,6 +54,20 @@ y: A | A B;

 AT_BISON_CHECK_CEX(
 [[input.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
+Shift/reduce conflict on token B:
+  Example              A . B C
+  Shift derivation
+    s
+    `-> y         c
+        `-> A . B `-> C
+  Reduce derivation
+    s
+    `-> a       x
+        `-> A . `-> B C
+
+input.y:4.4: warning: rule useless in parser due to conflicts [-Wother]
+]],
+[[input.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
 Shift/reduce conflict on token B:
  Example              A . B C
  Shift derivation     s -> [ y -> [ A . B ] c -> [ C ] ]
@@ -73,6 +96,38 @@ bc: B bc C | B C;

 AT_BISON_CHECK_CEX(
 [[input.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
+Shift/reduce conflict on token B:
+  Example              A . B C
+  Shift derivation
+    s
+    `-> ac
+        `-> A ac          C
+              `-> b
+                  `-> . B
+  Reduce derivation
+    s
+    `-> a       bc
+        `-> A . `-> B C
+
+Shift/reduce conflict on token B:
+  Example              A A . B B C C
+  Shift derivation
+    s
+    `-> ac
+        `-> A ac                        C
+              `-> A ac                C
+                    `-> b
+                        `-> . b
+                              `-> B B
+  Reduce derivation
+    s
+    `-> a             bc
+        `-> A a       `-> B bc      C
+              `-> A .       `-> B C
+
+input.y:6.4: warning: rule useless in parser due to conflicts [-Wother]
+]],
+[[input.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
 Shift/reduce conflict on token B:
  Example              A . B C
  Shift derivation     s -> [ ac -> [ A ac -> [ b -> [ . B ] ] C ] ]
@@ -107,6 +162,38 @@ xby: B | X xby Y;

 AT_BISON_CHECK_CEX(
 [[input.y: warning: 2 shift/reduce conflicts [-Wconflicts-sr]
+Shift/reduce conflict on token B:
+  Example              A . B
+  Shift derivation
+    s
+    `-> A xby
+          `-> . B
+  Reduce derivation
+    s
+    `-> ax          by
+        `-> A x     `-> B y
+              `-> .       `-> %empty
+
+Shift/reduce conflict on token B:
+  First example        A X . B Y $end
+  Shift derivation
+    $accept
+    `-> s                     $end
+        `-> A xby
+              `-> X xby     Y
+                    `-> . B
+  Second example       A X . B y $end
+  Reduce derivation
+    $accept
+    `-> s                             $end
+        `-> ax                by
+            `-> A x           `-> B y
+                  `-> X x
+                        `-> .
+
+input.y:5.4-9: warning: rule useless in parser due to conflicts [-Wother]
+]],
+[[input.y: warning: 2 shift/reduce conflicts [-Wconflicts-sr]
 Shift/reduce conflict on token B:
  Example              A . B
  Shift derivation     s -> [ A xby -> [ . B ] ]
@@ -142,6 +229,25 @@ bc: B C;

 AT_BISON_CHECK_CEX(
 [[input.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
+Shift/reduce conflict on token C:
+  First example        B . C $end
+  Shift derivation
+    $accept
+    `-> g                 $end
+        `-> x
+            `-> bc
+                `-> B . C
+  Second example       B . C D $end
+  Reduce derivation
+    $accept
+    `-> g                       $end
+        `-> x
+            `-> b       cd
+                `-> B . `-> C D
+
+input.y:6.4: warning: rule useless in parser due to conflicts [-Wother]
+]],
+[[input.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
 Shift/reduce conflict on token C:
  First example        B . C $end
  Shift derivation     $accept -> [ g -> [ x -> [ bc -> [ B . C ] ] ] $end ]
@@ -170,6 +276,25 @@ y: A A B;

 AT_BISON_CHECK_CEX(
 [[input.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
+Shift/reduce conflict on token A:
+  First example        A . A B $end
+  Shift derivation
+    $accept
+    `-> s                   $end
+        `-> t
+            `-> y
+                `-> A . A B
+  Second example       A . A $end
+  Reduce derivation
+    $accept
+    `-> s                             $end
+        `-> s               t
+            `-> t           `-> x
+                `-> x           `-> A
+                    `-> A .
+
+]],
+[[input.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
 Shift/reduce conflict on token A:
  First example        A . A B $end
  Shift derivation     $accept -> [ s -> [ t -> [ y -> [ A . A B ] ] ] $end ]
@@ -202,6 +327,36 @@ y: Y;

 AT_BISON_CHECK_CEX(
 [[input.y: warning: 2 shift/reduce conflicts [-Wconflicts-sr]
+Shift/reduce conflict on token A:
+  Example              b . A X X Y
+  Shift derivation
+    a
+    `-> s
+        `-> b . xx        y
+                `-> A X X `-> Y
+  Reduce derivation
+    a
+    `-> r       t
+        `-> b . `-> A x     xy
+                      `-> X `-> X Y
+
+Shift/reduce conflict on token X:
+  First example        A X . X
+  Shift derivation
+    a
+    `-> t
+        `-> A xx
+              `-> X . X
+  Second example       X . X xy
+  Reduce derivation
+    a
+    `-> x       t
+        `-> X . `-> X xy
+
+input.y:4.4: warning: rule useless in parser due to conflicts [-Wother]
+input.y:8.4: warning: rule useless in parser due to conflicts [-Wother]
+]],
+[[input.y: warning: 2 shift/reduce conflicts [-Wconflicts-sr]
 Shift/reduce conflict on token A:
  Example              b . A X X Y
  Shift derivation     a -> [ s -> [ b . xx -> [ A X X ] y -> [ Y ] ] ]
@@ -234,6 +389,19 @@ b : A | b;

 AT_BISON_CHECK_CEX(
 [[input.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
+Reduce/reduce conflict on token $end:
+  Example              A b .
+  First derivation
+    a
+    `-> A b .
+  Second derivation
+    a
+    `-> A b
+          `-> b .
+
+input.y:4.9: warning: rule useless in parser due to conflicts [-Wother]
+]],
+[[input.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
 Reduce/reduce conflict on token $end:
  Example              A b .
  First derivation     a -> [ A b . ]
@@ -260,6 +428,23 @@ b: D;

 AT_BISON_CHECK_CEX(
 [[input.y: warning: 2 reduce/reduce conflicts [-Wconflicts-rr]
+Reduce/reduce conflict on tokens A, C:
+  First example        D . A $end
+  First derivation
+    $accept
+    `-> s             $end
+        `-> a       A
+            `-> D .
+  Second example       B D . A $end
+  Second derivation
+    $accept
+    `-> s               $end
+        `-> B b       A
+              `-> D .
+
+input.y:5.4: warning: rule useless in parser due to conflicts [-Wother]
+]],
+[[input.y: warning: 2 reduce/reduce conflicts [-Wconflicts-rr]
 Reduce/reduce conflict on tokens A, C:
  First example        D . A $end
  First derivation     $accept -> [ s -> [ a -> [ D . ] A ] $end ]
@@ -288,6 +473,24 @@ i: X | i J K;
 AT_BISON_CHECK_CEX(
 [[input.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
 Shift/reduce conflict on token J:
+time limit exceeded: XXX
+  First example        H i . J K $end
+  Shift derivation
+    $accept
+    `-> a                 $end
+        `-> H i
+              `-> i . J K
+  Second example       H i . J $end
+  Reduce derivation
+    $accept
+    `-> s               $end
+        `-> a         J
+            `-> H i .
+
+input.y:4.4-6: warning: rule useless in parser due to conflicts [-Wother]
+]],
+[[input.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
+Shift/reduce conflict on token J:
 time limit exceeded: XXX
  First example        H i . J K $end
  Shift derivation     $accept -> [ a -> [ H i -> [ i . J K ] ] $end ]
@@ -319,6 +522,37 @@ b: A B C | A B D;

 AT_BISON_CHECK_CEX(
 [[input.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
+Shift/reduce conflict on token B:
+  Example              N A . B C
+  Shift derivation
+    s
+    `-> n
+        `-> N b
+              `-> A . B C
+  Reduce derivation
+    s
+    `-> n               C
+        `-> N a       B
+              `-> A .
+
+Shift/reduce conflict on token B:
+  Example              N N A . B D C
+  Shift derivation
+    s
+    `-> n
+        `-> N n                 C
+              `-> N b
+                    `-> A . B D
+  Reduce derivation
+    s
+    `-> n                       C
+        `-> N n               D
+              `-> N a       B
+                    `-> A .
+
+input.y:5.4: warning: rule useless in parser due to conflicts [-Wother]
+]],
+[[input.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
 Shift/reduce conflict on token B:
  Example              N A . B C
  Shift derivation     s -> [ n -> [ N b -> [ A . B C ] ] ]
@@ -355,6 +589,38 @@ C : A c A;

 AT_BISON_CHECK_CEX(
 [[input.y: warning: 4 reduce/reduce conflicts [-Wconflicts-rr]
+Reduce/reduce conflict on tokens b, c:
+  Example              B . b c
+  First derivation
+    S
+    `-> B                        C
+        `-> A       b A          `-> A          c A
+            `-> B .   `-> %empty     `-> %empty   `-> %empty
+  Second derivation
+    S
+    `-> B C
+          `-> A                          c A
+              `-> B                        `-> %empty
+                  `-> A     b A
+                      `-> .   `-> %empty
+
+Reduce/reduce conflict on tokens b, c:
+  Example              C . c b
+  First derivation
+    S
+    `-> C                        B
+        `-> A       c A          `-> A          b A
+            `-> C .   `-> %empty     `-> %empty   `-> %empty
+  Second derivation
+    S
+    `-> C B
+          `-> A                          b A
+              `-> C                        `-> %empty
+                  `-> A     c A
+                      `-> .   `-> %empty
+
+]],
+[[input.y: warning: 4 reduce/reduce conflicts [-Wconflicts-rr]
 Reduce/reduce conflict on tokens b, c:
  Example              B . b c
  First derivation     S -> [ B -> [ A -> [ B . ] b A -> [ ] ] C -> [ A -> [ ] c A -> [ ] ] ]
@@ -387,6 +653,136 @@ d : a | c A | d;
 AT_BISON_CHECK_CEX(
 [[input.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
 input.y: warning: 6 reduce/reduce conflicts [-Wconflicts-rr]
+Reduce/reduce conflict on token A:
+  First example        . c A A $end
+  First derivation
+    $accept
+    `-> a                   $end
+        `-> b     d
+            `-> . `-> c A A
+  Second example       . c A A $end
+  Second derivation
+    $accept
+    `-> a                   $end
+        `-> c     d
+            `-> . `-> c A A
+
+Reduce/reduce conflict on token A:
+time limit exceeded: XXX
+  First example        b . c A A $end
+  First derivation
+    $accept
+    `-> a                             $end
+        `-> b d
+              `-> a
+                  `-> b     d
+                      `-> . `-> c A A
+  Second example       b . A $end
+  Second derivation
+    $accept
+    `-> a                 $end
+        `-> b d
+              `-> c     A
+                  `-> .
+
+Reduce/reduce conflict on token A:
+time limit exceeded: XXX
+  First example        c . c A A $end
+  First derivation
+    $accept
+    `-> a                             $end
+        `-> c d
+              `-> a
+                  `-> b     d
+                      `-> . `-> c A A
+  Second example       c . A $end
+  Second derivation
+    $accept
+    `-> a                 $end
+        `-> c d
+              `-> c     A
+                  `-> .
+
+Shift/reduce conflict on token A:
+time limit exceeded: XXX
+  First example        b c . A
+  Shift derivation
+    a
+    `-> b d
+          `-> c . A
+  Second example       b c . c A A $end
+  Reduce derivation
+    $accept
+    `-> a                                       $end
+        `-> b d
+              `-> a
+                  `-> c d
+                        `-> a
+                            `-> b     d
+                                `-> . `-> c A A
+
+Reduce/reduce conflict on token A:
+  First example        b c . c A A $end
+  First derivation
+    $accept
+    `-> a                                       $end
+        `-> b d
+              `-> a
+                  `-> c d
+                        `-> a
+                            `-> b     d
+                                `-> . `-> c A A
+  Second example       b c . A $end
+  Second derivation
+    $accept
+    `-> a                           $end
+        `-> b d
+              `-> a
+                  `-> c d
+                        `-> c     A
+                            `-> .
+
+Shift/reduce conflict on token A:
+  First example        b c . A
+  Shift derivation
+    a
+    `-> b d
+          `-> c . A
+  Second example       b c . A $end
+  Reduce derivation
+    $accept
+    `-> a                           $end
+        `-> b d
+              `-> a
+                  `-> c d
+                        `-> c     A
+                            `-> .
+
+Reduce/reduce conflict on token $end:
+  Example              b d .
+  First derivation
+    a
+    `-> b d .
+  Second derivation
+    a
+    `-> b d
+          `-> d .
+
+Reduce/reduce conflict on token $end:
+  Example              c d .
+  First derivation
+    a
+    `-> c d .
+  Second derivation
+    a
+    `-> c d
+          `-> d .
+
+input.y:5.4: warning: rule useless in parser due to conflicts [-Wother]
+input.y:6.15: warning: rule useless in parser due to conflicts [-Wother]
+]],
+[[input.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
+input.y: warning: 6 reduce/reduce conflicts [-Wconflicts-rr]
 Reduce/reduce conflict on token A:
  First example        . c A A $end
  First derivation     $accept -> [ a -> [ b -> [ . ] d -> [ c A A ] ] $end ]
@@ -461,6 +857,21 @@ i: %empty | i J;

 AT_BISON_CHECK_CEX(
 [[input.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
+Shift/reduce conflict on token J:
+  Example              H i J . J J
+  Shift derivation
+    s
+    `-> a             J
+        `-> H i J . J
+  Reduce derivation
+    s
+    `-> a
+        `-> H i         J J
+              `-> i J .
+
+input.y:5.13-15: warning: rule useless in parser due to conflicts [-Wother]
+]],
+[[input.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
 Shift/reduce conflict on token J:
  Example              H i J . J J
  Shift derivation     s -> [ a -> [ H i J . J ] J ]
@@ -492,6 +903,21 @@ d: D;

 AT_BISON_CHECK_CEX(
 [[input.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
+Shift/reduce conflict on token D:
+  Example              A a . D
+  Shift derivation
+    s
+    `-> A a d
+            `-> . D
+  Reduce derivation
+    s
+    `-> A a a             d
+            `-> b         `-> D
+                `-> c
+                    `-> .
+
+]],
+[[input.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
 Shift/reduce conflict on token D:
  Example              A a . D
  Shift derivation     s -> [ A a d -> [ . D ] ]
@@ -521,6 +947,24 @@ d: D;

 AT_BISON_CHECK_CEX(
 [[input.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
+Shift/reduce conflict on token D:
+  First example        A a . D $end
+  Shift derivation
+    $accept
+    `-> s               $end
+        `-> A a d
+                `-> . D
+  Second example       A a . D E $end
+  Reduce derivation
+    $accept
+    `-> s                             $end
+        `-> A a a             d     E
+                `-> b         `-> D
+                    `-> c
+                        `-> .
+
+]],
+[[input.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
 Shift/reduce conflict on token D:
  First example        A a . D $end
  Shift derivation     $accept -> [ s -> [ A a d -> [ . D ] ] $end ]
--- a/tests/diagnostics.at
+++ b/tests/diagnostics.at
@@ -28,6 +28,8 @@ m4_pushdef([AT_TEST],
 AT_SETUP([$1])
 AT_KEYWORDS([diagnostics])

+m4_if(m4_index([$1], [Counterexample]), [-1], [], [AT_KEYWORDS([cex])])
+
 # We need UTF-8 support for correct screen-width computation of UTF-8
 # characters.  Skip the test if not available.
 locale=`locale -a | $EGREP '^en_US\.(UTF-8|utf8)$' | sed 1q`
@@ -535,32 +537,114 @@ exp
 ]],
 [1],
 [[input.y: <error>error:</error> shift/reduce conflicts: 4 found, 0 expected
-Shift/reduce conflict on token "+":
+Shift/reduce conflict on token "+":
+  Example              <cex-0><cex-leaf>exp</cex-leaf> <cex-leaf>"+"</cex-leaf><cex-1> <cex-leaf>exp</cex-leaf> <cex-dot>•</cex-dot> <cex-leaf>"+"</cex-leaf> <cex-leaf>exp</cex-leaf></cex-1></cex-0>
+  Shift derivation
+    <cex-0><cex-step>exp</cex-step></cex-0>
    <cex-0><cex-step>↳ <cex-leaf>exp</cex-leaf><cex-leaf> "+"</cex-leaf><cex-1><cex-step> exp</cex-step></cex-1></cex-step></cex-0>
-  Shift derivation     <cex-0><cex-step>exp → [ </cex-step><cex-leaf>exp</cex-leaf> <cex-leaf>"+"</cex-leaf><cex-1> <cex-step>exp → [ </cex-step><cex-leaf>exp</cex-leaf> <cex-dot>•</cex-dot> <cex-leaf>"+"</cex-leaf> <cex-leaf>exp</cex-leaf><cex-step> ]</cex-step></cex-1><cex-step> ]</cex-step></cex-0>
+    <cex-1><cex-step>          ↳ <cex-leaf>exp</cex-leaf><cex-dot> •</cex-dot><cex-leaf> "+"</cex-leaf><cex-leaf> exp</cex-leaf></cex-step></cex-1>
+  Example              <cex-0><cex-1><cex-leaf>exp</cex-leaf> <cex-leaf>"+"</cex-leaf> <cex-leaf>exp</cex-leaf> <cex-dot>•</cex-dot></cex-1> <cex-leaf>"+"</cex-leaf> <cex-leaf>exp</cex-leaf></cex-0>
+  Reduce derivation
+    <cex-0><cex-step>exp</cex-step></cex-0>
    <cex-0><cex-step>↳ <cex-1><cex-step>exp</cex-step></cex-1><cex-leaf>             "+"</cex-leaf><cex-leaf> exp</cex-leaf></cex-step></cex-0>
    <cex-1><cex-step>  ↳ <cex-leaf>exp</cex-leaf><cex-leaf> "+"</cex-leaf><cex-leaf> exp</cex-leaf><cex-dot> •</cex-dot></cex-step></cex-1>

-Shift/reduce conflict on token "else":
+Shift/reduce conflict on token "else":
+  Example              <cex-0><cex-leaf>"if"</cex-leaf> <cex-leaf>exp</cex-leaf> <cex-leaf>"then"</cex-leaf><cex-1> <cex-leaf>"if"</cex-leaf> <cex-leaf>exp</cex-leaf> <cex-leaf>"then"</cex-leaf> <cex-leaf>exp</cex-leaf> <cex-dot>•</cex-dot> <cex-leaf>"else"</cex-leaf> <cex-leaf>exp</cex-leaf></cex-1></cex-0>
+  Shift derivation
+    <cex-0><cex-step>exp</cex-step></cex-0>
    <cex-0><cex-step>↳ <cex-leaf>"if"</cex-leaf><cex-leaf> exp</cex-leaf><cex-leaf> "then"</cex-leaf><cex-1><cex-step> exp</cex-step></cex-1></cex-step></cex-0>
-  Shift derivation     <cex-0><cex-step>exp → [ </cex-step><cex-leaf>"if"</cex-leaf> <cex-leaf>exp</cex-leaf> <cex-leaf>"then"</cex-leaf><cex-1> <cex-step>exp → [ </cex-step><cex-leaf>"if"</cex-leaf> <cex-leaf>exp</cex-leaf> <cex-leaf>"then"</cex-leaf> <cex-leaf>exp</cex-leaf> <cex-dot>•</cex-dot> <cex-leaf>"else"</cex-leaf> <cex-leaf>exp</cex-leaf><cex-step> ]</cex-step></cex-1><cex-step> ]</cex-step></cex-0>
+    <cex-1><cex-step>                  ↳ <cex-leaf>"if"</cex-leaf><cex-leaf> exp</cex-leaf><cex-leaf> "then"</cex-leaf><cex-leaf> exp</cex-leaf><cex-dot> •</cex-dot><cex-leaf> "else"</cex-leaf><cex-leaf> exp</cex-leaf></cex-step></cex-1>
+  Example              <cex-0><cex-leaf>"if"</cex-leaf> <cex-leaf>exp</cex-leaf> <cex-leaf>"then"</cex-leaf><cex-1> <cex-leaf>"if"</cex-leaf> <cex-leaf>exp</cex-leaf> <cex-leaf>"then"</cex-leaf> <cex-leaf>exp</cex-leaf> <cex-dot>•</cex-dot></cex-1> <cex-leaf>"else"</cex-leaf> <cex-leaf>exp</cex-leaf></cex-0>
+  Reduce derivation
+    <cex-0><cex-step>exp</cex-step></cex-0>
    <cex-0><cex-step>↳ <cex-leaf>"if"</cex-leaf><cex-leaf> exp</cex-leaf><cex-leaf> "then"</cex-leaf><cex-1><cex-step> exp</cex-step></cex-1><cex-leaf>                     "else"</cex-leaf><cex-leaf> exp</cex-leaf></cex-step></cex-0>
    <cex-1><cex-step>                  ↳ <cex-leaf>"if"</cex-leaf><cex-leaf> exp</cex-leaf><cex-leaf> "then"</cex-leaf><cex-leaf> exp</cex-leaf><cex-dot> •</cex-dot></cex-step></cex-1>

-Shift/reduce conflict on token "+":
+Shift/reduce conflict on token "+":
+  Example              <cex-0><cex-leaf>"if"</cex-leaf> <cex-leaf>exp</cex-leaf> <cex-leaf>"then"</cex-leaf><cex-1> <cex-leaf>exp</cex-leaf> <cex-dot>•</cex-dot> <cex-leaf>"+"</cex-leaf> <cex-leaf>exp</cex-leaf></cex-1></cex-0>
+  Shift derivation
+    <cex-0><cex-step>exp</cex-step></cex-0>
    <cex-0><cex-step>↳ <cex-leaf>"if"</cex-leaf><cex-leaf> exp</cex-leaf><cex-leaf> "then"</cex-leaf><cex-1><cex-step> exp</cex-step></cex-1></cex-step></cex-0>
-  Shift derivation     <cex-0><cex-step>exp → [ </cex-step><cex-leaf>"if"</cex-leaf> <cex-leaf>exp</cex-leaf> <cex-leaf>"then"</cex-leaf><cex-1> <cex-step>exp → [ </cex-step><cex-leaf>exp</cex-leaf> <cex-dot>•</cex-dot> <cex-leaf>"+"</cex-leaf> <cex-leaf>exp</cex-leaf><cex-step> ]</cex-step></cex-1><cex-step> ]</cex-step></cex-0>
+    <cex-1><cex-step>                  ↳ <cex-leaf>exp</cex-leaf><cex-dot> •</cex-dot><cex-leaf> "+"</cex-leaf><cex-leaf> exp</cex-leaf></cex-step></cex-1>
+  Example              <cex-0><cex-1><cex-leaf>"if"</cex-leaf> <cex-leaf>exp</cex-leaf> <cex-leaf>"then"</cex-leaf> <cex-leaf>exp</cex-leaf> <cex-dot>•</cex-dot></cex-1> <cex-leaf>"+"</cex-leaf> <cex-leaf>exp</cex-leaf></cex-0>
+  Reduce derivation
+    <cex-0><cex-step>exp</cex-step></cex-0>
    <cex-0><cex-step>↳ <cex-1><cex-step>exp</cex-step></cex-1><cex-leaf>                     "+"</cex-leaf><cex-leaf> exp</cex-leaf></cex-step></cex-0>
    <cex-1><cex-step>  ↳ <cex-leaf>"if"</cex-leaf><cex-leaf> exp</cex-leaf><cex-leaf> "then"</cex-leaf><cex-leaf> exp</cex-leaf><cex-dot> •</cex-dot></cex-step></cex-1>

-Shift/reduce conflict on token "+":
+Shift/reduce conflict on token "+":
+  Example              <cex-0><cex-leaf>"if"</cex-leaf> <cex-leaf>exp</cex-leaf> <cex-leaf>"then"</cex-leaf> <cex-leaf>exp</cex-leaf> <cex-leaf>"else"</cex-leaf><cex-1> <cex-leaf>exp</cex-leaf> <cex-dot>•</cex-dot> <cex-leaf>"+"</cex-leaf> <cex-leaf>exp</cex-leaf></cex-1></cex-0>
+  Shift derivation
+    <cex-0><cex-step>exp</cex-step></cex-0>
    <cex-0><cex-step>↳ <cex-leaf>"if"</cex-leaf><cex-leaf> exp</cex-leaf><cex-leaf> "then"</cex-leaf><cex-leaf> exp</cex-leaf><cex-leaf> "else"</cex-leaf><cex-1><cex-step> exp</cex-step></cex-1></cex-step></cex-0>
-  Shift derivation     <cex-0><cex-step>exp → [ </cex-step><cex-leaf>"if"</cex-leaf> <cex-leaf>exp</cex-leaf> <cex-leaf>"then"</cex-leaf> <cex-leaf>exp</cex-leaf> <cex-leaf>"else"</cex-leaf><cex-1> <cex-step>exp → [ </cex-step><cex-leaf>exp</cex-leaf> <cex-dot>•</cex-dot> <cex-leaf>"+"</cex-leaf> <cex-leaf>exp</cex-leaf><cex-step> ]</cex-step></cex-1><cex-step> ]</cex-step></cex-0>
+    <cex-1><cex-step>                             ↳ <cex-leaf>exp</cex-leaf><cex-dot> •</cex-dot><cex-leaf> "+"</cex-leaf><cex-leaf> exp</cex-leaf></cex-step></cex-1>
+  Example              <cex-0><cex-1><cex-leaf>"if"</cex-leaf> <cex-leaf>exp</cex-leaf> <cex-leaf>"then"</cex-leaf> <cex-leaf>exp</cex-leaf> <cex-leaf>"else"</cex-leaf> <cex-leaf>exp</cex-leaf> <cex-dot>•</cex-dot></cex-1> <cex-leaf>"+"</cex-leaf> <cex-leaf>exp</cex-leaf></cex-0>
+  Reduce derivation
+    <cex-0><cex-step>exp</cex-step></cex-0>
    <cex-0><cex-step>↳ <cex-1><cex-step>exp</cex-step></cex-1><cex-leaf>                                "+"</cex-leaf><cex-leaf> exp</cex-leaf></cex-step></cex-0>
    <cex-1><cex-step>  ↳ <cex-leaf>"if"</cex-leaf><cex-leaf> exp</cex-leaf><cex-leaf> "then"</cex-leaf><cex-leaf> exp</cex-leaf><cex-leaf> "else"</cex-leaf><cex-leaf> exp</cex-leaf><cex-dot> •</cex-dot></cex-step></cex-1>

 ]])
+
+
+AT_TEST([[Deep Counterexamples]],
+[[%expect 0
+%%
+exp: x1 e1 foo1 x1 | y1 e2 bar1 y1
+foo1: foo2
+foo2: foo3
 foo3: x1 foo4
+foo4: "quuux"
+
+bar1: bar2
+bar2: bar3
+bar3: y1 bar4
+bar4: "quuux"
+
+x1: x2
+x2: x3
+x3: "X"
+
+y1: y2
+y2: y3
+y3: "X"
+
+e1:
+e2:
+]],
+[1],
+[[input.y:30.4: <warning>warning:</warning> empty rule without %empty [<warning>-Wempty-rule</warning>]
+   30 | e1:
+      |    <warning>^</warning>
+      |    <fixit-insert>%empty</fixit-insert>
+input.y:31.4: <warning>warning:</warning> empty rule without %empty [<warning>-Wempty-rule</warning>]
+   31 | e2:
+      |    <warning>^</warning>
+      |    <fixit-insert>%empty</fixit-insert>
+input.y: <error>error:</error> reduce/reduce conflicts: 1 found, 0 expected
+Reduce/reduce conflict on token "X":
+  Example              <cex-0><cex-1><cex-2><cex-3><cex-leaf>"X"</cex-leaf> <cex-dot>•</cex-dot></cex-3></cex-2></cex-1><cex-4></cex-4><cex-5><cex-6><cex-7><cex-8><cex-9><cex-10> <cex-leaf>"X"</cex-leaf></cex-10></cex-9></cex-8><cex-11> <cex-leaf>"quuux"</cex-leaf></cex-11></cex-7></cex-6></cex-5><cex-12><cex-13><cex-14> <cex-leaf>"X"</cex-leaf></cex-14></cex-13></cex-12></cex-0>
+  First derivation
+    <cex-0><cex-step>exp</cex-step></cex-0>
+    <cex-0><cex-step>↳ <cex-1><cex-step>x1</cex-step></cex-1><cex-4><cex-step>          e1</cex-step></cex-4><cex-5><cex-step>  foo1</cex-step></cex-5><cex-12><cex-step>                      x1</cex-step></cex-12></cex-step></cex-0>
+    <cex-1><cex-step>  ↳ <cex-2><cex-step>x2</cex-step></cex-2></cex-step></cex-1><cex-4><cex-step>        ↳ ε</cex-step></cex-4><cex-5><cex-step> ↳ <cex-6><cex-step>foo2</cex-step></cex-6></cex-step></cex-5><cex-12><cex-step>                    ↳ <cex-13><cex-step>x2</cex-step></cex-13></cex-step></cex-12>
+    <cex-2><cex-step>    ↳ <cex-3><cex-step>x3</cex-step></cex-3></cex-step></cex-2><cex-6><cex-step>            ↳ <cex-7><cex-step>foo3</cex-step></cex-7></cex-step></cex-6><cex-13><cex-step>                    ↳ <cex-14><cex-step>x3</cex-step></cex-14></cex-step></cex-13>
+    <cex-3><cex-step>      ↳ <cex-leaf>"X"</cex-leaf><cex-dot> •</cex-dot></cex-step></cex-3><cex-7><cex-step>         ↳ <cex-8><cex-step>x1</cex-step></cex-8><cex-11><cex-step>        foo4</cex-step></cex-11></cex-step></cex-7><cex-14><cex-step>          ↳ <cex-leaf>"X"</cex-leaf></cex-step></cex-14>
+    <cex-8><cex-step>                        ↳ <cex-9><cex-step>x2</cex-step></cex-9></cex-step></cex-8><cex-11><cex-step>      ↳ <cex-leaf>"quuux"</cex-leaf></cex-step></cex-11>
+    <cex-9><cex-step>                          ↳ <cex-10><cex-step>x3</cex-step></cex-10></cex-step></cex-9>
+    <cex-10><cex-step>                            ↳ <cex-leaf>"X"</cex-leaf></cex-step></cex-10>
+  Example              <cex-0><cex-1><cex-2><cex-3><cex-leaf>"X"</cex-leaf> <cex-dot>•</cex-dot></cex-3></cex-2></cex-1><cex-4></cex-4><cex-5><cex-6><cex-7><cex-8><cex-9><cex-10> <cex-leaf>"X"</cex-leaf></cex-10></cex-9></cex-8><cex-11> <cex-leaf>"quuux"</cex-leaf></cex-11></cex-7></cex-6></cex-5><cex-12><cex-13><cex-14> <cex-leaf>"X"</cex-leaf></cex-14></cex-13></cex-12></cex-0>
+  Second derivation
+    <cex-0><cex-step>exp</cex-step></cex-0>
+    <cex-0><cex-step>↳ <cex-1><cex-step>y1</cex-step></cex-1><cex-4><cex-step>          e2</cex-step></cex-4><cex-5><cex-step>  bar1</cex-step></cex-5><cex-12><cex-step>                      y1</cex-step></cex-12></cex-step></cex-0>
+    <cex-1><cex-step>  ↳ <cex-2><cex-step>y2</cex-step></cex-2></cex-step></cex-1><cex-4><cex-step>        ↳ ε</cex-step></cex-4><cex-5><cex-step> ↳ <cex-6><cex-step>bar2</cex-step></cex-6></cex-step></cex-5><cex-12><cex-step>                    ↳ <cex-13><cex-step>y2</cex-step></cex-13></cex-step></cex-12>
+    <cex-2><cex-step>    ↳ <cex-3><cex-step>y3</cex-step></cex-3></cex-step></cex-2><cex-6><cex-step>            ↳ <cex-7><cex-step>bar3</cex-step></cex-7></cex-step></cex-6><cex-13><cex-step>                    ↳ <cex-14><cex-step>y3</cex-step></cex-14></cex-step></cex-13>
+    <cex-3><cex-step>      ↳ <cex-leaf>"X"</cex-leaf><cex-dot> •</cex-dot></cex-step></cex-3><cex-7><cex-step>         ↳ <cex-8><cex-step>y1</cex-step></cex-8><cex-11><cex-step>        bar4</cex-step></cex-11></cex-step></cex-7><cex-14><cex-step>          ↳ <cex-leaf>"X"</cex-leaf></cex-step></cex-14>
+    <cex-8><cex-step>                        ↳ <cex-9><cex-step>y2</cex-step></cex-9></cex-step></cex-8><cex-11><cex-step>      ↳ <cex-leaf>"quuux"</cex-leaf></cex-step></cex-11>
+    <cex-9><cex-step>                          ↳ <cex-10><cex-step>y3</cex-step></cex-10></cex-step></cex-9>
+    <cex-10><cex-step>                            ↳ <cex-leaf>"X"</cex-leaf></cex-step></cex-10>
+
 input.y: <warning>warning:</warning> fix-its can be applied.  Rerun with option '--update'. [<warning>-Wother</warning>]
 ]])

--- a/tests/report.at
+++ b/tests/report.at
@@ -1539,39 +1539,74 @@ AT_CHECK([LC_ALL="$locale" bison -fno-caret -o input.cc -rall -Wcex --graph=inpu
 input.y: warning: 3 reduce/reduce conflicts [-Wconflicts-rr]
 Shift/reduce conflict on token "⊕":
  Example              exp "+" exp • "⊕" exp
-  Shift derivation     exp → [ exp "+" exp → [ exp • "⊕" exp ] ]
-  Reduce derivation    exp → [ exp → [ exp "+" exp • ] "⊕" exp ]
+  Shift derivation
+    exp
+    ↳ exp "+" exp
+              ↳ exp • "⊕" exp
+  Reduce derivation
+    exp
+    ↳ exp             "⊕" exp
+      ↳ exp "+" exp •

 Reduce/reduce conflict on tokens $end, "+", "⊕":
  Example              exp "+" exp •
-  First derivation     exp → [ exp "+" exp • ]
-  Second derivation    exp → [ exp "+" exp • ]
+  First derivation
+    exp
+    ↳ exp "+" exp •
+  Second derivation
+    exp
+    ↳ exp "+" exp •

 Shift/reduce conflict on token "⊕":
  Example              exp "+" exp • "⊕" exp
-  Shift derivation     exp → [ exp "+" exp → [ exp • "⊕" exp ] ]
-  Reduce derivation    exp → [ exp → [ exp "+" exp • ] "⊕" exp ]
+  Shift derivation
+    exp
+    ↳ exp "+" exp
+              ↳ exp • "⊕" exp
+  Reduce derivation
+    exp
+    ↳ exp             "⊕" exp
+      ↳ exp "+" exp •

 Shift/reduce conflict on token "⊕":
  Example              exp "⊕" exp • "⊕" exp
-  Shift derivation     exp → [ exp "⊕" exp → [ exp • "⊕" exp ] ]
-  Reduce derivation    exp → [ exp → [ exp "⊕" exp • ] "⊕" exp ]
+  Shift derivation
+    exp
+    ↳ exp "⊕" exp
+              ↳ exp • "⊕" exp
+  Reduce derivation
+    exp
+    ↳ exp             "⊕" exp
+      ↳ exp "⊕" exp •

 Shift/reduce conflict on token "+":
  Example              exp "⊕" exp • "+" exp
-  Shift derivation     exp → [ exp "⊕" exp → [ exp • "+" exp ] ]
-  Reduce derivation    exp → [ exp → [ exp "⊕" exp • ] "+" exp ]
+  Shift derivation
+    exp
+    ↳ exp "⊕" exp
+              ↳ exp • "+" exp
+  Reduce derivation
+    exp
+    ↳ exp             "+" exp
+      ↳ exp "⊕" exp •

 Shift/reduce conflict on token "+":
  Example              exp "⊕" exp • "+" exp
-  Shift derivation     exp → [ exp "⊕" exp → [ exp • "+" exp ] ]
-  Reduce derivation    exp → [ exp → [ exp "⊕" exp • ] "+" exp ]
+  Shift derivation
+    exp
+    ↳ exp "⊕" exp
+              ↳ exp • "+" exp
+  Reduce derivation
+    exp
+    ↳ exp             "+" exp
+      ↳ exp "⊕" exp •

 input.y:6.3-13: warning: rule useless in parser due to conflicts [-Wother]
 ]])

 # Check the contents of the report.
-AT_CHECK([cat input.output], [],
+# FIXME: Avoid trailing white spaces.
+AT_CHECK([sed -e 's/ *$//' input.output], [],
 [[Rules useless in parser due to conflicts

    3 exp: exp "+" exp
@@ -1714,22 +1749,38 @@ State 7
        2 exp: exp "+" exp •
        1 exp: exp • "⊕" exp
      Example                  exp "+" exp • "⊕" exp
-      Shift derivation         exp → [ exp "+" exp → [ exp • "⊕" exp ] ]
-      Reduce derivation        exp → [ exp → [ exp "+" exp • ] "⊕" exp ]
+      Shift derivation
+        exp
+        ↳ exp "+" exp
+                  ↳ exp • "⊕" exp
+      Reduce derivation
+        exp
+        ↳ exp             "⊕" exp
+          ↳ exp "+" exp •

    Reduce/reduce conflict on tokens $end, "+", "⊕":
        2 exp: exp "+" exp •
        3 exp: exp "+" exp •
      Example                  exp "+" exp •
-      First derivation         exp → [ exp "+" exp • ]
-      Second derivation        exp → [ exp "+" exp • ]
+      First derivation
+        exp
+        ↳ exp "+" exp •
+      Second derivation
+        exp
+        ↳ exp "+" exp •

    Shift/reduce conflict on token "⊕":
        3 exp: exp "+" exp •
        1 exp: exp • "⊕" exp
      Example                  exp "+" exp • "⊕" exp
-      Shift derivation         exp → [ exp "+" exp → [ exp • "⊕" exp ] ]
-      Reduce derivation        exp → [ exp → [ exp "+" exp • ] "⊕" exp ]
+      Shift derivation
+        exp
+        ↳ exp "+" exp
+                  ↳ exp • "⊕" exp
+      Reduce derivation
+        exp
+        ↳ exp             "⊕" exp
+          ↳ exp "+" exp •



@@ -1751,22 +1802,40 @@ State 8
        1 exp: exp "⊕" exp •
        1 exp: exp • "⊕" exp
      Example                  exp "⊕" exp • "⊕" exp
-      Shift derivation         exp → [ exp "⊕" exp → [ exp • "⊕" exp ] ]
-      Reduce derivation        exp → [ exp → [ exp "⊕" exp • ] "⊕" exp ]
+      Shift derivation
+        exp
+        ↳ exp "⊕" exp
+                  ↳ exp • "⊕" exp
+      Reduce derivation
+        exp
+        ↳ exp             "⊕" exp
+          ↳ exp "⊕" exp •

    Shift/reduce conflict on token "+":
        1 exp: exp "⊕" exp •
        2 exp: exp • "+" exp
      Example                  exp "⊕" exp • "+" exp
-      Shift derivation         exp → [ exp "⊕" exp → [ exp • "+" exp ] ]
-      Reduce derivation        exp → [ exp → [ exp "⊕" exp • ] "+" exp ]
+      Shift derivation
+        exp
+        ↳ exp "⊕" exp
+                  ↳ exp • "+" exp
+      Reduce derivation
+        exp
+        ↳ exp             "+" exp
+          ↳ exp "⊕" exp •

    Shift/reduce conflict on token "+":
        1 exp: exp "⊕" exp •
        3 exp: exp • "+" exp
      Example                  exp "⊕" exp • "+" exp
-      Shift derivation         exp → [ exp "⊕" exp → [ exp • "+" exp ] ]
-      Reduce derivation        exp → [ exp → [ exp "⊕" exp • ] "+" exp ]
+      Shift derivation
+        exp
+        ↳ exp "⊕" exp
+                  ↳ exp • "+" exp
+      Reduce derivation
+        exp
+        ↳ exp             "+" exp
+          ↳ exp "⊕" exp •

 ]])