val expr : t -> t expr
(** Equality over formulae *)
-val st : t -> StateSet.t * StateSet.t
-(** states occuring left and right, positively or negatively *)
+val st : t -> StateSet.t * StateSet.t * StateSet.t
+(** states occuring self, left and right, positively or negatively *)
val compare : t -> t -> int
(** Comparison of formulae *)