proposition: Lemma free0 k : free Unit k = Unit :> U.
proof: Proof. by rewrite !pcmE /= !umEX /UM.free /UM.empty rem_empty. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path bigop) pcm(options prelude finmap seqperm pred seqext) pcm(pcm morphism)