multistart: check duplicates

* src/symlist.h, src/symlist.c (symbol_list_find_symbol)
(symbol_list_last): New.
(symbol_list_append): Use symbol_list_last.
* src/reader.c (grammar_start_symbols_add): Check and discard duplicates.
* tests/input.at (Duplicate %start symbol): New.
* tests/reduce.at (Bad start symbols): Add the multistart keyword.

src/symlist.c

@@ -91,6 +91,16 @@ symbol_list_type_set (symbol_list *syms, uniqstr type_name)
 }
 
 
+symbol_list *
+symbol_list_find_symbol (symbol_list *l, const symbol *sym)
+{
+  for (/* Nothing. */; l && l->content.sym; l = l->next)
+    if (l->content.sym == sym)
+      return l;
+  return NULL;
+}
+
+
 /*-----------------------------------------------------------------------.
 | Print this list, for which every content_type must be SYMLIST_SYMBOL.  |
 `-----------------------------------------------------------------------*/
@@ -129,23 +139,29 @@ symbol_list_prepend (symbol_list *list, symbol_list *node)
 }
 
 
-/*-------------------------.
-| Append NODE to the LIST. |
-`-------------------------*/
+symbol_list *
+symbol_list_last (symbol_list *list)
+{
+  if (!list)
+    return NULL;
+  symbol_list *next = list;
+  while (next->next)
+    next = next->next;
+  return next;
+}
 
 symbol_list *
 symbol_list_append (symbol_list *list, symbol_list *node)
 {
-  if (!list)
-    return node;
-  symbol_list *next = list;
-  while (next->next)
-    next = next->next;
-  next->next = node;
+  if (list)
+    symbol_list_last (list)->next = node;
+  else
+    list = node;
   return list;
 }
 
 
+
 /*-----------------------------------------------.
 | Free the LIST, but not the items it contains.  |
 `-----------------------------------------------*/