proposition: Lemma unitbP (x : U) : reflect (x = Unit) (unitb x).
proof: Proof. by case: U x=>V [b][u]. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq bigop fintype finset) pcm(options axioms prelude seqperm pred seqext)