proposition: Lemma gp_undef : gapless undef.
proof: Proof. by move=>k; rewrite lastkey_undef; case: k. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path interval) pcm(options prelude pred finmap rtc seqext useqord uslice uconsec)