End Uniq.

Lemma length_split (A B: Type) (a b: A) (lk: list B) l1 l2:
length lk = length ((a:: l1) ++ b :: l2)%list ->
exists k1, exists k2, exists lk1, exists lk2,
lk = ((k1::lk1) ++ k2::lk2)%list /\ length lk1 = length l1 /\ length lk2 = length l2.