(* Definition of the expandM modality *)
Definition knot_rel (k1 k2:knot) :=
let (n,f) := unsquash k1 in
let (n',f') := unsquash k2 in
n = n' /\ Rel assert f f'.

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