Definition stratify (n:nat) : Mor predS (projT2 (sinv n)) :=
    Build_Mor predS (projT2 (sinv n)) (stratify0 n) (stratify_mor n).

(* Specifications: *)
Lemma decompose_nat : forall (x y:nat), { m:nat & y = (m + S x) } + { ge x y }.