proposition: Lemma invr_out : {in [pred x | val x \isn't a unit], invU =1 id}.
proof: Proof. by move=> x /[!inE] xNU; apply: val_inj; rewrite SubK invr_out. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)