cex: introduce the parse simulator

* src/derivation.h, src/derivation.c,
* src/parse-simulation.h, src/parse-simulation.c: New.

src/derivation.h

@@ -0,0 +1,46 @@
+/* Counterexample derivation trees
+
+   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 DERIVATION_H
+# define DERIVATION_H
+
+# include <gl_list.h>
+
+# include "gram.h"
+
+/* Derivations are trees of symbols such that each non terminal's
+   children are symbols that produce that nonterminal if they are
+   relevant to the counterexample. The leaves of a derivation form a
+   counterexample when printed. */
+
+typedef struct derivation
+{
+  symbol_number sym;
+  gl_list_t children;
+} derivation;
+
+derivation *derivation_new (symbol_number sym, gl_list_t children);
+size_t derivation_size (const derivation *deriv);
+void derivation_print (const derivation *deriv, FILE *f);
+void derivation_print_leaves (const derivation *deriv, FILE *f);
+void derivation_free (derivation *deriv);
+
+const derivation *derivation_dot (void);
+
+#endif /* DERIVATION_H */