proposition: Lemma freeND k f : k \notin dom f -> free f k = f.
proof: Proof. by move=>W; rewrite -[f]unitR freeUnL // free0. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path bigop) pcm(options prelude finmap seqperm pred seqext) pcm(pcm morphism)