proposition: Lemma lastkeyU u h t : t \in dom h -> last_key (upd t u h) = last_key h.
proof: Proof. by case/um_eta=>v [_ E]; rewrite E updPtUn (lastkeyPtUnE v). Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path interval) pcm(options prelude pred finmap rtc seqext useqord uslice uconsec)