proposition: Lemma mem_xx t1 t2 (ks : seq A) k : uniq ks -> reflect ([/\ k \in ks, t1 <=[ks] k & k <=[ks] t2]) (k \in &=ks `[t1, t2]).
proof: Proof. by move=>U; rewrite eqslice_mem_uniq // in_itv; apply: and3P. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path interval order) pcm(options prelude ordtype seqext slice useqord)