throws Throwable
  public Value eval(Env env)    throws Throwable  {    String name = _expr.evalString(env);    // return env.include(_dir, name);    if (_dir != null)      return env.include(_dir, name, _isRequire, true);    else if (_isRequire)      return env.require_once(name);    else      return env.include_once(name);  }