Goal:
  
  P : E.t -> Prop
  P_Proper : Proper (E.eq ==> impl) P
  s : t
  x : elt
  xs : list elt
  IH : Forall P xs <-> (forall x : elt, InA E.eq x xs -> P x)
  ============================
   (Forall P (x :: xs) <-> (forall x0 : elt, InA E.eq x0 (x :: xs) -> P x0))


DEBUG:Going to execute:
setoid_rewrite Forall_cons_iff