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