Returns a nested iterable by removing one element at a time, starting from the beginning.
1, 2, 3.tails = (1, 2, 3, 2, 3, 3, []).
1, 2, 3
2, 3
3
[].tails returns [].
[]
Set<Set<E>> get tails => h.tailsIterable(this).toNestedSet();