proposition: Lemma find_none k f : find k f = None <-> k \notin dom f.
proof: Proof. by case: dom_find=>// v ->. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path bigop) pcm(options prelude finmap seqperm pred seqext) pcm(pcm morphism)