public bool?AreNonNullUnequal(MaybeX <T> rhs) =>
 this.HasValue && rhs.HasValue ? !this._value.Equals(rhs._value)
                                       : null as bool?;
 public bool Equals(MaybeX <T> other) =>
 _value != null ? other._value != null && (_value == other._value || _value.Equals(other._value))
                        : other._value == null;