@Override
  public boolean equals(Object obj) {
    if (this == obj)
      return true;
    if (obj == null)
      return false;
    if (getClass() != obj.getClass())
      return false;
    PairImpl<?> other = (PairImpl<?>) obj;
    return Tuples.equals(_first, other._first)
        && Tuples.equals(_second, other._second);
  }