proposition: Lemma mxxyM t x y : valid (x \+ (y \+ mx t)) -> x \+ y = Unit.
proof: Proof. by rewrite joinA=>/mxxM. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq) pcm(options prelude pred) pcm(pcm morphism natmap)