induction 1; intuition; simpl in *.
intuition.
intuition.
pairInv.
trivial.
Qed.

(* Specifications: *)
Lemma in_zip_swap :
forall (A B : Set)(lsa : list A)(lsb : list B) a b,
length lsa = length lsb ->
In (a, b) (zip lsa lsb) ->
In (b, a) (zip lsb lsa).