Fixpoint sinv (n: nat) : Type :=
    match n with
      | O => unit
      | S n => prodT (sinv n) ((F (sinv n) * other) -> T)
    end.Fixpoint floor (m:nat) (n:nat) (p:sinv (m+n)) : sinv n :=
    match m as m' return forall (p : sinv (m'+n)), sinv n with
    | O => fun p => p
    | S m' => fun p => floor m' n (fst p)
    end p.Definition knot := { n:nat & F (sinv n) }.Definition predicate := knot * other -> T.Fixpoint stratify (n:nat) (Q:predicate) {struct n} : sinv n :=
    match n as n' return sinv n' with
    | O => tt
    | S n' => ( stratify n' Q, fun v => Q (existT (F oo sinv) n' (fst v),snd v) )
    end.

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