proposition: Lemma mxPJ x y : mxjoin_spec x y (x \+ y) x y.
proof: Proof. by case: x y=>[||x][||y]; constructor. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq) pcm(options prelude pred) pcm(pcm morphism natmap)