code: function tokenStackLex() {
      var token;
      token = tstack.pop() || lexer.lex() || EOF;
      // if token isn't its numeric value, convert
      if (typeof token !== 'number') {
          if (token instanceof Array) {
              tstack = token;
              token = tstack.pop();
          }
          token = self.symbols_[token] || token;
      }
      return token;
  }
docs: lex function that supports token stacks