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 knot_level : forall k:knot,
level k = fst (unsquash k).