Goal:
    P : elt -> elt -> Prop  s : t
  ============================
   (For_all (fun v1 : elt => For_all (P v1) s) s <->
    (forall v1 v2 : elt, In v1 s -> In v2 s -> P v1 v2))


DEBUG:Going to execute:
cbv[For_all]