proposition: Lemma mem2_splice p1 p3 x y p2 : mem2 (p1 ++ p3) x y -> mem2 (p1 ++ p2 ++ p3) x y.
proof: Proof. by rewrite !mem2_cat mem_cat andb_orr orbC => /or3P[]->; rewrite ?orbT. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq)