proposition: Lemma joinxK' (x x1 x2 : U) : x \+ x1 = x \+ x2 -> valid (x \+ x1) -> x1 = x2.
proof: Proof. by move=>/[swap]; exact: joinxK. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq) pcm(options pred prelude) pcm(pcm unionmap natmap)