cex: introduce counterexample search

* src/counterexample.h, src/counterexample.c: New.

src/counterexample.h

@@ -0,0 +1,35 @@
+/* Conflict counterexample generation
+
+   Copyright (C) 2020 Free Software Foundation, Inc.
+
+   This file is part of Bison, the GNU Compiler Compiler.
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#ifndef COUNTEREXAMPLE_H
+# define COUNTEREXAMPLE_H
+
+# include "state-item.h"
+
+void counterexample_init (void);
+void counterexample_free (void);
+
+void counterexample_report_shift_reduce (state_item_number itm1,
+                                         state_item_number itm2,
+                                         symbol_number next_sym);
+
+void counterexample_report_reduce_reduce (state_item_number itm1,
+                                          state_item_number itm2,
+                                          bitset conflict_syms);
+#endif /* COUNTEREXAMPLE_H */