cex: also include the counterexamples in the report

The report is the best place to show the details about
counterexamples, since we have the state right under the nose.

For instance:

State 7

    1 exp: exp . "⊕" exp
    2    | exp . "+" exp
    2    | exp "+" exp .  [$end, "+", "⊕"]
    3    | exp . "+" exp
    3    | exp "+" exp .  [$end, "+", "⊕"]

    "⊕"  shift, and go to state 6

    $end      reduce using rule 2 (exp)
    $end      [reduce using rule 3 (exp)]
    "+"       reduce using rule 2 (exp)
    "+"       [reduce using rule 3 (exp)]
    "⊕"       [reduce using rule 2 (exp)]
    "⊕"       [reduce using rule 3 (exp)]
    $default  reduce using rule 2 (exp)

    Conflict between rule 2 and token "+" resolved as reduce (%left "+").

    Shift/reduce conflict on token "⊕":
        2 exp: exp "+" exp .
        1 exp: exp . "⊕" exp
      Example                  exp "+" exp • "⊕" exp
      First derivation         exp ::=[ exp ::=[ exp "+" exp • ] "⊕" exp ]
      Example                  exp "+" exp • "⊕" exp
      Second 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 • ]
      Example                  exp "+" exp •
      Second derivation        exp ::=[ exp "+" exp • ]

    Shift/reduce conflict on token "⊕":
        3 exp: exp "+" exp .
        1 exp: exp . "⊕" exp
      Example                  exp "+" exp • "⊕" exp
      First derivation         exp ::=[ exp ::=[ exp "+" exp • ] "⊕" exp ]
      Example                  exp "+" exp • "⊕" exp
      Second derivation        exp ::=[ exp "+" exp ::=[ exp • "⊕" exp ] ]

* src/conflicts.h, src/conflicts.c (has_conflicts): New.
* src/counterexample.h, src/counterexample.c (print_counterexample):
Add a `prefix` argument.
(counterexample_report_shift_reduce)
(counterexample_report_reduce_reduce): Show the items when there's a
prefix.
* src/state-item.h, src/state-item.c (print_state_item):
Add a `prefix` argument.
* src/derivation.h, src/derivation.c (derivation_print)
(derivation_print_leaves): Add a prefix argument.
* src/print.c (print_state): When -Wcex is enabled, show the
conflicts.
* tests/report.at: Adjust.

tests/report.at

@@ -1148,7 +1148,7 @@ AT_CLEANUP
 
 AT_SETUP([Reports with conflicts])
 
-AT_KEYWORDS([report])
+AT_KEYWORDS([cex report])
 
 # We need UTF-8 support for correct screen-width computation of UTF-8
 # characters.  Skip the test if not available.
@@ -1167,10 +1167,33 @@ exp
 | "Ñùṃéℝô"
 ]])
 
-AT_CHECK([LC_ALL="$locale" $5 bison -fno-caret -o input.cc -rall --graph=input.gv --xml input.y], [], [],
+AT_CHECK([LC_ALL="$locale" bison -fno-caret -o input.cc -rall -Wcex --graph=input.gv --xml input.y], [], [],
 [[input.y: warning: 3 shift/reduce conflicts [-Wconflicts-sr]
 input.y: warning: 3 reduce/reduce conflicts [-Wconflicts-rr]
-input.y: warning: rerun with option '-Wcounterexamples' to generate conflict counterexamples [-Wother]
+Shift/reduce conflict on token "⊕":
+  Example              exp "+" exp • "⊕" exp
+  First derivation     exp ::=[ exp ::=[ exp "+" exp • ] "⊕" exp ]
+  Example              exp "+" exp • "⊕" exp
+  Second derivation    exp ::=[ exp "+" exp ::=[ exp • "⊕" exp ] ]
+
+Reduce/reduce conflict on tokens $end, "+", "⊕":
+  Example              exp "+" exp •
+  First derivation     exp ::=[ exp "+" exp • ]
+  Example              exp "+" exp •
+  Second derivation    exp ::=[ exp "+" exp • ]
+
+Shift/reduce conflict on token "⊕":
+  Example              exp "+" exp • "⊕" exp
+  First derivation     exp ::=[ exp ::=[ exp "+" exp • ] "⊕" exp ]
+  Example              exp "+" exp • "⊕" exp
+  Second derivation    exp ::=[ exp "+" exp ::=[ exp • "⊕" exp ] ]
+
+Shift/reduce conflict on token "⊕":
+  Example              exp "⊕" exp • "⊕" exp
+  First derivation     exp ::=[ exp ::=[ exp "⊕" exp • ] "⊕" exp ]
+  Example              exp "⊕" exp • "⊕" exp
+  Second derivation    exp ::=[ exp "⊕" exp ::=[ exp • "⊕" exp ] ]
+
 input.y:6.3-13: warning: rule useless in parser due to conflicts [-Wother]
 ]])
 
@@ -1314,6 +1337,31 @@ State 7
 
     Conflict between rule 2 and token "+" resolved as reduce (%left "+").
 
+    Shift/reduce conflict on token "⊕":
+        2 exp: exp "+" exp .
+        1 exp: exp . "⊕" exp
+      Example                  exp "+" exp • "⊕" exp
+      First derivation         exp ::=[ exp ::=[ exp "+" exp • ] "⊕" exp ]
+      Example                  exp "+" exp • "⊕" exp
+      Second 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 • ]
+      Example                  exp "+" exp •
+      Second derivation        exp ::=[ exp "+" exp • ]
+
+    Shift/reduce conflict on token "⊕":
+        3 exp: exp "+" exp .
+        1 exp: exp . "⊕" exp
+      Example                  exp "+" exp • "⊕" exp
+      First derivation         exp ::=[ exp ::=[ exp "+" exp • ] "⊕" exp ]
+      Example                  exp "+" exp • "⊕" exp
+      Second derivation        exp ::=[ exp "+" exp ::=[ exp • "⊕" exp ] ]
+
+
 
 State 8
 
@@ -1328,6 +1376,15 @@ State 8
     "+"       [reduce using rule 1 (exp)]
     "⊕"       [reduce using rule 1 (exp)]
     $default  reduce using rule 1 (exp)
+
+    Shift/reduce conflict on token "⊕":
+        1 exp: exp "⊕" exp .
+        1 exp: exp . "⊕" exp
+      Example                  exp "⊕" exp • "⊕" exp
+      First derivation         exp ::=[ exp ::=[ exp "⊕" exp • ] "⊕" exp ]
+      Example                  exp "⊕" exp • "⊕" exp
+      Second derivation        exp ::=[ exp "⊕" exp ::=[ exp • "⊕" exp ] ]
+
 ]])