proposition: Lemma subdom_valid x y : {subset dom x <= [predC dom y]} -> valid x -> valid y -> valid (x \+ y).
proof: Proof. by move=>H X Y; case: validUn; rewrite ?X ?Y=>// k /H /negbTE /= ->. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path bigop) pcm(options prelude finmap seqperm pred seqext) pcm(pcm morphism)