new file mode 100644
--- /dev/null
+++ b/src/pt-eval.h
@@ -0,0 +1,185 @@
+/*
+
+Copyright (C) 2009 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 3 of the License, 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, see
+<http://www.gnu.org/licenses/>.
+
+*/
+
+#if !defined (octave_tree_eval_h)
+#define octave_tree_eval_h 1
+
+#include <stack>
+#include <string>
+
+#include "comment-list.h"
+#include "oct-obj.h"
+#include "pt-walk.h"
+
+class tree_expression;
+
+// How to evaluate the code that the parse trees represent.
+
+class
+tree_evaluator : public tree_walker
+{
+public:
+
+  typedef void (*decl_elt_init_fcn) (tree_decl_elt&);
+
+  tree_evaluator (bool in_function_or_script_body_arg = false)
+    : in_function_or_script_body (in_function_or_script_body_arg) { }
+
+  ~tree_evaluator (void) { }
+
+  void reset (void)
+  {
+    in_function_or_script_body = false;
+  }
+
+  void visit_anon_fcn_handle (tree_anon_fcn_handle&);
+
+  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_global_command (tree_global_command&);
+
+  void visit_static_command (tree_static_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_script (octave_user_script&);
+
+  void visit_octave_user_function (octave_user_function&);
+
+  void visit_octave_user_function_header (octave_user_function&);
+
+  void visit_octave_user_function_trailer (octave_user_function&);
+
+  void visit_function_def (tree_function_def&);
+
+  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_matrix (tree_matrix&);
+
+  void visit_cell (tree_cell&);
+
+  void visit_multi_assignment (tree_multi_assignment&);
+
+  void visit_no_op_command (tree_no_op_command&);
+
+  void visit_constant (tree_constant&);
+
+  void visit_fcn_handle (tree_fcn_handle&);
+
+  void visit_parameter_list (tree_parameter_list&);
+
+  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_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&);
+
+  void visit_do_until_command (tree_do_until_command&);
+
+  static int debug_line (void) { return db_line; }
+
+  static int debug_column (void) { return db_column; }
+
+  // If > 0, stop executing at the (N-1)th stopping point, counting
+  //         from the the current execution point in the current frame.
+  //
+  // If < 0, stop executing at the next possible stopping point.
+  static int dbstep_flag;
+
+  // The number of the stack frame we are currently debugging.
+  static size_t current_frame;
+
+  static bool debug_mode;
+
+private:
+
+  bool in_function_or_script_body;
+
+  void do_decl_init_list (decl_elt_init_fcn fcn,
+			  tree_decl_init_list *init_list);
+
+  void do_breakpoint (tree_statement& stmt) const;
+
+  void do_breakpoint (bool is_breakpoint, int l, int c,
+		      bool is_end_of_fcn_or_script = false) const;
+
+  static int db_line;
+  static int db_column;
+
+  // No copying!
+
+  tree_evaluator (const tree_evaluator&);
+
+  tree_evaluator& operator = (const tree_evaluator&);
+};
+
+extern tree_evaluator *current_evaluator;
+
+#endif
+
+/*
+;;; Local Variables: ***
+;;; mode: C++ ***
+;;; End: ***
+*/