new file mode 100644
--- /dev/null
+++ b/src/pt-check.h
@@ -0,0 +1,147 @@
+/*
+
+Copyright (C) 1996, 1997 John W. Eaton
+
+This file is part of Octave.
+
+Octave 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 2, or (at your option) any
+later version.
+
+Octave 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 Octave; see the file COPYING.  If not, write to the Free
+Software Foundation, 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.
+
+*/
+
+#if !defined (octave_tree_checker_h)
+#define octave_tree_checker_h 1
+
+#if defined (__GNUG__)
+#pragma interface
+#endif
+
+#include "pt-walk.h"
+
+// How to check the semantics of the code that the parse trees represent.
+
+class
+tree_checker : public tree_walker
+{
+public:
+
+  tree_checker (void)
+    : do_lvalue_check (false) { }
+
+  ~tree_checker (void) { }
+
+  void visit_argument_list (tree_argument_list&);
+
+  void visit_binary_expression (tree_binary_expression&);
+
+  void visit_break_command (tree_break_command&);
+
+  void visit_colon_expression (tree_colon_expression&);
+
+  void visit_continue_command (tree_continue_command&);
+
+  void visit_decl_command (tree_decl_command&);
+
+  void visit_decl_elt (tree_decl_elt&);
+
+  void visit_decl_init_list (tree_decl_init_list&);
+
+  void visit_simple_for_command (tree_simple_for_command&);
+
+  void visit_complex_for_command (tree_complex_for_command&);
+
+  void visit_octave_user_function (octave_user_function&);
+
+  void visit_identifier (tree_identifier&);
+
+  void visit_if_clause (tree_if_clause&);
+
+  void visit_if_command (tree_if_command&);
+
+  void visit_if_command_list (tree_if_command_list&);
+
+  void visit_index_expression (tree_index_expression&);
+
+  void visit_indirect_ref (tree_indirect_ref&);
+
+  void visit_matrix (tree_matrix&);
+
+  void visit_multi_assignment (tree_multi_assignment&);
+
+  void visit_no_op_command (tree_no_op_command&);
+
+  void visit_constant (tree_constant&);
+
+  void visit_parameter_list (tree_parameter_list&);
+
+  void visit_plot_command (tree_plot_command&);
+
+  void visit_plot_limits (plot_limits&);
+
+  void visit_plot_range (plot_range&);
+
+  void visit_postfix_expression (tree_postfix_expression&);
+
+  void visit_prefix_expression (tree_prefix_expression&);
+
+  void visit_return_command (tree_return_command&);
+
+  void visit_return_list (tree_return_list&);
+
+  void visit_simple_assignment (tree_simple_assignment&);
+
+  void visit_statement (tree_statement&);
+
+  void visit_statement_list (tree_statement_list&);
+
+  void visit_subplot (subplot&);
+
+  void visit_subplot_list (subplot_list&);
+
+  void visit_subplot_style (subplot_style&);
+
+  void visit_subplot_using (subplot_using&);
+
+  void visit_switch_case (tree_switch_case&);
+
+  void visit_switch_case_list (tree_switch_case_list&);
+
+  void visit_switch_command (tree_switch_command&);
+
+  void visit_try_catch_command (tree_try_catch_command&);
+
+  void visit_unwind_protect_command (tree_unwind_protect_command&);
+
+  void visit_while_command (tree_while_command&);
+
+private:
+
+  bool do_lvalue_check;
+
+  void gripe (const string& msg, int line);
+
+  // No copying!
+
+  tree_checker (const tree_checker&);
+
+  tree_checker& operator = (const tree_checker&);
+};
+
+#endif
+
+/*
+;;; Local Variables: ***
+;;; mode: C++ ***
+;;; End: ***
+*/