Section SubDaddsmx.
Variables m m1 m2 n : nat.
Variables (A : 'M[F]_(m, n)) (B1 : 'M[F]_(m1, n)) (B2 : 'M[F]_(m2, n)).
Variant sub_daddsmx_spec : Prop :=
SubDaddsmxSpec A1 A2 of (A1 <= B1)%MS & (A2 <= B2)%MS & A = A1 + A2
& forall C1 C2, (C1 <= B1)%MS -> (C2 <= B2)%MS ->
A = C1 + C2 -> C1 = A1 /\ C2 = A2.

Lemma sub_daddsmx : (B1 :&: B2 = 0)%MS -> (A <= B1 + B2)%MS -> sub_daddsmx_spec.