Notation permutation := (permutation _ eq_dec).Notation list_contents := (list_contents _ eq_dec).

Lemma permut_length_2 : forall a1 b1 a2 b2, permutation (a1 :: b1 :: nil) (a2 :: b2 :: nil) -> (a1=a2) /\ (b1=b2) \/ (a1=b2) /\ (a2=b1).