public void Flatten_WithEmptyEnumerable_ExpectsSameEnumerable()
 => Utilities.AssertManySequencesEqual(
     ENMRBL_EMPTY.Flatten(),
     ENMRBL_EMPTY);
 public void Flatten_WithEmptyEnumerableAndNullProvider_Throws()
 => Throws <ArgumentNullException>(
     () => ENMRBL_EMPTY.Flatten(null).ToList());