proposition: Lemma lastkey_omf h : last_key (f h) <= last_key h.
proof: Proof. by case: f=>s [m H] /=; rewrite H lastkey_omap. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path interval) pcm(options prelude pred finmap rtc seqext useqord uslice uconsec)