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.
0.094dev (2025-06-18 15:08:51 GIT hash 89cffc23ae669b0898a5564fefbf793fcb8e5ca7 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.