create a tuple of two Sequences by splitting this at the given index, i.e., a Sequence of length 'at' and one of length 'count-at'.
at may be <= 0 or >= count, in which case the resulting tuple will be the (empty list, Sequence.this.asList) or (Sequence.this.asList, empty list), resp.
a Sequence of length 'at' and one of length 'count-at'.
at may be <= 0 or >= count, in which case the resulting tuple will be the
(empty list, Sequence.this.asList) or (Sequence.this.asList, empty list), resp.