Module Type KNOT.
Declare Module TF:TY_FUNCTOR.
Parameter knot : Type.
Parameter ag_knot : ageable knot.
Existing Instance ag_knot.
Existing Instance ag_prod.
Definition predicate := (knot * other) -> T.
Parameter squash : (nat * F predicate) -> knot.
Parameter unsquash : knot -> (nat * F predicate).
Definition approx (n:nat) (p:predicate) : predicate :=
fun w => if le_gt_dec n (level w) then T_bot else p w.

Axiom squash_unsquash : forall x, squash (unsquash x) = x.
Axiom unsquash_squash : forall n x', unsquash (squash (n,x')) = (n,fmap F (approx n) x').