catch (Throwable t) { rethrow(t);
catch (RuntimeException e) { rethrow(e, RuntimeException.class);
  public Value execute(Env env)    throws Throwable  {    try {      if (_expr != null) {        return _expr.evalRef(env);      }      else        return NullValue.NULL;    }    catch (Throwable t) {      rethrow(t);    }        return null;  }