proposition: Lemma opt_undef_join (x : optPCM U) : opt_undef \+ x = opt_undef.
proof: Proof. by []. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq bigop fintype finset) pcm(options axioms prelude seqperm pred seqext)