proposition: Lemma lastval0 : last_val Unit = None.
proof: Proof. by rewrite /last_val lastkey0 find0E. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path interval) pcm(options prelude pred finmap rtc seqext useqord uslice uconsec)