Module Type KNOT__COVARIANT_HERED_PROP.
Declare Module KI : KNOT_INPUT__COVARIANT_HERED_PROP.
Parameter knot : Type.
Parameter ageable_knot : ageable knot.
Existing Instance ageable_knot.
Parameter squash : (nat * F (pred knot)) -> knot.
Parameter unsquash : knot -> (nat * F (pred knot)).
Parameter approx : nat -> pred knot -> pred knot.

Axiom knot_age1 : forall (k:knot),
age1 k =
match unsquash k with
| (O,_) => None
| (S n,x) => Some (squash (n,x))
end.