let p_subst alpha x v p =
match v with
| Fol.Tvar y when (Fol.Var.equal x y) -> p (* v is equal to x *)
| (Fol.Tconst _ | Fol.Tvar _ | Fol.Tapp(_,[]))
when Wp_parameters.Simpl.get () ->
Fol.pred_replace alpha x v p
| _ -> Fol.p_let x v p