Definition squash (x:nat * F predicate) : knot :=
    match x with (n,y) => existT (F oo sinv) n (fmap F (stratify n) y) end.Definition unsquash (x:knot) : (nat * F predicate) :=
    match x with existT _ n y => (n, fmap F (unstratify n) y) end.Definition def_knot_level (k:knot) := fst (unsquash k).Definition def_knot_age1 (k:knot) : option knot :=
    match unsquash k with
    | (O,_) => None
    | (S n,x) => Some (squash (n,x))
    end.Definition def_knot_unage (k:knot) : knot :=
    match unsquash k with
    | (n,x) => squash (S n,x)
    end.Definition approx (n:nat) (p:predicate) : predicate :=
     fun w => if le_gt_dec n (def_knot_level (fst w)) then T_bot else p w.

Lemma squash_unsquash : forall x, squash (unsquash x) = x.