(* Token factories *)
Parameter isTokenFactory : t -> nat -> Prop.
Parameter isToken : t -> nat -> Prop.
Parameter create_token : nat -> t -> (t*t).

Axiom create_token_correct : forall fac fac' tok x n,
create_token n fac = (fac',tok) ->
isTokenFactory fac x ->
isTokenFactory fac' (n+x) /\
isToken tok n /\
join fac' tok fac.