Inductive SubMaskSpec (p q : positive) : mask -> Prop :=
| SubIsNul : p = q -> SubMaskSpec p q IsNul
| SubIsPos : forall r, q + r = p -> SubMaskSpec p q (IsPos r)
| SubIsNeg : forall r, p + r = q -> SubMaskSpec p q IsNeg.

Lemma sub_mask_add_diag_l p q : sub_mask (p+q) p = IsPos q.