Goal:
  
  P : E.t -> Prop
  s : t
  P_dec : forall x : E.t, {P x} + {~ P x}
  P_Proper : Proper (E.eq ==> impl) P
  ============================
   ({Exists P s} + {~ Exists P s})


DEBUG:Going to execute:
destruct (Exists_dec P (elements s) P_dec) as [H| H]