proposition: Lemma freeL k f1 f2 : valid (f1 \+ f2) -> k \in dom f1 -> free (f1 \+ f2) k = free f1 k \+ f2.
proof: Proof. by move=>W /(dom_inNL W) D; rewrite freeUnR. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path bigop) pcm(options prelude finmap seqperm pred seqext) pcm(pcm morphism)