proposition: Lemma validUnF k f1 f2 : valid (f1 \+ f2) -> valid (f1 \+ free f2 k).
proof: Proof. by rewrite !(joinC f1); apply: validFUn. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path bigop) pcm(options prelude finmap seqperm pred seqext) pcm(pcm morphism)