Goal:
    P : E.t -> Prop  P_Proper : Proper (E.eq ==> impl) P  s : t
  ============================
   (List.Exists P (elements s) <-> (exists x : elt, In x s /\ P x))


DEBUG:Going to execute:
<coq-core.plugins.ltac::setoid_rewrite@0> $1 $2