proposition: Lemma mulmx_lsub m n p k (A : 'M[R]_(m, n)) (B : 'M[R]_(n, p + k)) : A *m lsubmx B = (lsubmx (A *m B)).
proof: Proof. by apply/matrixP=> i j; rewrite !mxE; apply: eq_bigr => l //= _; rewrite mxE. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq path) mathcomp(ssralg fintype finfun perm matrix bigop zmodp mxalgebra) mathcomp(choice poly polydiv mxpoly binomial)