proposition: Lemma joinpf x y : valid (x \+ y) -> D x y -> f x \+ f y = f (x \+ y).
proof: Proof. by move=>*; rewrite pfjoin. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(eqtype fintype) pcm(options axioms prelude) pcm(pcm)