fact: Definition lower_part_mx m n (M : 'M[R]_(m,n)) := \matrix_(i, j) (M i j *+ (j <= i)).
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq path) mathcomp(ssralg fintype perm poly mxpoly finfun tuple) mathcomp(matrix bigop zmodp polydiv) ssrcomplements dvdring