* @return true if this is equal to the other year-month
   */
  @Override
  public boolean equals(Object obj) {

    if (this == obj) {
      return true;
    }
    if (obj instanceof YearMonth) {
      YearMonth other = (YearMonth) obj;
      return this.year == other.year && this.month == other.month;
    }
    return false;
  }