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