Section StructuralLinear.
Canonical diag_mx_additive n := Additive (@diag_mx_is_linear n).
Canonical diag_mx_linear n := Linear (@diag_mx_is_linear n).
(* Matrix multiplication using bigops. *)
Fact mulmx_key : unit. Proof. by []. Qed.
Definition mulmx {m n p} (A : 'M_(m, n)) (B : 'M_(n, p)) : 'M[R]_(m, p) :=
\matrix[mulmx_key]_(i, k) \sum_j (A i j * B j k).
Local Notation "A *m B" := (mulmx A B) : ring_scope.
(* Right scaling associativity requires a commutative ring *)

Lemma mul_delta_mx m n p (j : 'I_n) (i : 'I_m) (k : 'I_p) :
delta_mx i j *m delta_mx j k = delta_mx i k.