throws Throwable
  public Value add(Value rValue)    throws Throwable  {    rValue = rValue.toValue();    if (! (rValue instanceof ArrayValue))      return copy();    ArrayValue rArray = (ArrayValue) rValue;    ArrayValue result = new ArrayValueImpl(rArray);    for (Entry entry = getHead(); entry != null; entry = entry._next) {      result.put(entry.getKey(), entry.getValue());    }    return result;  }