fact: Definition mxring_id m n (R : 'A_(m, n)) e := [/\ e != 0, e \in R, forall A, A \in R -> e *m A = A & forall A, A \in R -> A *m e = A]%MS.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype finfun bigop finset fingroup perm order) mathcomp(div prime binomial ssralg finalg zmodp matrix)