proposition: Lemma valid_subdom x y : valid (x \+ y) -> {subset dom x <= [predC dom y]}.
proof: Proof. by case: validUn. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path bigop) pcm(options prelude finmap seqperm pred seqext) pcm(pcm morphism)