Section Compat.
End Compat.

Lemma In_In_list_decompose {A} (x y : A) l : In x l -> In y l ->
x = y \/
exists l1 l2 l3, (l = l1 ++ x :: l2 ++ y :: l3) \/ (l = l1 ++ y :: l2 ++ x :: l3).