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 (2025-07-10 15:41:47 GIT hash eafa42ae743de61b62f958948d7abb5fada7756f 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.