proposition: Lemma uundef_join_fin x : uundef \+ x = uundef.
proof: Proof. by apply: fin_ext=>a; rewrite !sel_fin undef_join. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq bigop fintype finset) pcm(options axioms prelude seqperm pred seqext)