throws Throwable
  public boolean evalBoolean(Env env)    throws Throwable  {    Value value = _value.eval(env);    if (! (value instanceof ArrayValue))      return false;    ArrayValue array = (ArrayValue) value;    if (! array.hasCurrent())      return false;    if (_keyVar != null)      _keyVar.evalAssign(env, array.key());    if (_valueVar != null)      _valueVar.evalAssign(env, array.current());    array.next();    return true;  }