proposition: Lemma mem_split (x : A) s : x \in s -> exists s1 s2, s = s1 ++ [:: x] ++ s2.
proof: Proof. by case/in_split=>s1 [s2][H _]; exists s1, s2. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat seq eqtype path fintype finfun tuple interval perm fingroup) pcm(options prelude ordtype seqext slice)