proposition: Lemma join0E f1 f2 : f1 \+ f2 = Unit <-> f1 = Unit /\ f2 = Unit.
proof: Proof. by rewrite !unitbE um_unitbUn; case: andP. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path bigop) pcm(options prelude finmap seqperm pred seqext) pcm(pcm morphism)