proposition: Lemma unitr_algid1 A u : u \in A -> u \is a GRing.unit -> algid A = 1.
proof: Proof. by move=> Eu /mulrI; apply; rewrite mulr1 algidr. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(choice fintype div tuple finfun bigop ssralg) mathcomp(finalg zmodp matrix vector poly)