Canonical scalar_mx_additive := Additive scalar_mx_is_additive.

(* Specifications: *)
Lemma row1 i : row i 1%:M = delta_mx 0 i.
Lemma is_scalar_mxP A : reflect (exists a, A = a%:M) (is_scalar_mx A).
Lemma scalar_mx_is_scalar a : is_scalar_mx a%:M.
Lemma mx0_is_scalar : is_scalar_mx 0.
Lemma mx11_scalar (A : 'M_1) : A = (A 0 0)%:M.
Lemma scalar_mx_block n1 n2 a : a%:M = block_mx a%:M 0 0 a%:M :> 'M_(n1 + n2).