public mod parseExpression() {
   mod tree = null;
   PythonParser parser = setupParser(false);
   try {
     PythonParser.eval_input_return r = parser.eval_input();
     tree = (mod) r.tree;
   } catch (RecognitionException e) {
     // XXX: this can't happen.  Need to strip the throws from antlr
     //     generated code.
   }
   return tree;
 }