public bool Equals(NonEmptyEnumerable <T>?other) =>
 other switch
 {
     null => false,
 public NonEmptyEnumerable <T> Concat(NonEmptyEnumerable <T> enumerable) =>
 new NonEmptyEnumerable <T>(_head, _tail.Concat(enumerable));
 public static bool Equals(NonEmptyEnumerable <T> first, NonEmptyEnumerable <T> second) =>
 first.Equals(second);