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.as_list) or (Sequence.this.as_list, empty list), resp.
0.095dev (2025-08-15 12:02:22 GIT hash 301b5b75e77076d091b38f555473f9f0e31e5b5c built by fridi@fzen)
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.as_list) or (Sequence.this.as_list, empty list), resp.