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);