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


DEBUG:Going to execute:
intros H H'
DEBUG LTAC Evaluated term: Exists_alt_iff
DEBUG LTAC Evaluated term: H