fact: Definition is_triangular_mx m n (M : 'M[R]_(m,n)) := upper_triangular_mx M || lower_triangular_mx M.
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