proposition: Lemma unitL (x : U) : Unit \+ x = x.
proof: Proof. rewrite /Unit/join. by case: U x=>tp [v j z Cj Aj Uj *]; apply: Uj. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq bigop fintype finset) pcm(options axioms prelude seqperm pred seqext)