Goal:
  
  P : E.t -> Prop
  P_Proper : Proper (E.eq ==> impl) P
  s : t
  x : elt
  xs : list elt
  H0 : forall x : elt,
       (fun x0 : elt => InA E.eq x0 xs /\ P x0) x -> List.Exists P xs
  x0 : elt
  H2 : P x0
  H1 : InA E.eq x0 xs
  H3 : forall y : E.t, E.eq x0 y -> impl (P x0) (P y)
  H4 : E.eq x0 x -> impl (P x0) (P x)
  H5 : List.Exists P xs
  x1 : elt
  H : InA E.eq x1 xs
  H6 : P x1
  ============================
   (P x)


DEBUG:Going to execute:
match goal with
           | |- ?X1 -> ?X2 => intro
           | |- ~ _ => unfold not at 1; intro
           end