public virtual_parse_stack(Stack shadowing_stack) throws java.lang.Exception
    {
      /* sanity check */
      if (shadowing_stack == null)
	throw new Exception(
	  "Internal parser error: attempt to create null virtual stack");

      /* set up our internals */
      real_stack = shadowing_stack;
      vstack     = new Stack();
      real_next  = 0;

      /* get one element onto the virtual portion of the stack */
      get_from_real();
    }