create a tuple of two Sequences by splitting this at

either

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.

or

at first result of condition function