--- a/src/pt-exp.h
+++ b/src/pt-exp.h
@@ -28,6 +28,7 @@
 
 class octave_value;
 class octave_lvalue;
+class symbol_table;
 
 #include "pt.h"
 
@@ -46,6 +47,8 @@
 
   virtual bool has_magic_end (void) const = 0;
 
+  virtual tree_expression *dup (symbol_table *) = 0;
+
   virtual bool is_constant (void) const { return false; }
 
   virtual bool is_matrix_constant (void) const { return false; }
@@ -104,6 +107,13 @@
       return this;
     }
 
+  virtual void copy_base (tree_expression& e)
+    {
+      num_parens = e.num_parens;
+      postfix_indexed = e.postfix_indexed;
+      print_flag = e.print_flag;
+    }
+
 protected:
 
   // A count of the number of times this expression appears directly