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