proposition: Lemma validL f1 f2 : valid (f1 \+ f2) -> valid f1.
proof: Proof. by rewrite !umEX; case: (UMC.from f1). Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path bigop) pcm(options prelude finmap seqperm pred seqext) pcm(pcm morphism)