if (obj == null || ! (obj instanceof BigInteger))
if (x.words == null && y.words == null) return x.ival == y.ival; if (x.words == null || y.words == null || x.ival != y.ival)
  public boolean equals(Object obj)  {    if (obj == null || ! (obj instanceof BigInteger))      return false;    return BigInteger.equals(this, (BigInteger) obj);  }