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


DEBUG:Going to execute:
<Coq.Init.Tauto.is_unit_or_eq> flags X1; cut X2;
            [ intro; clear id | cut X1; [ exact id | constructor 1; fail ] ]
Level 0: Backtrace:

P is used in hypothesis P_Proper.
Level 0: Backtrace:

P is used in hypothesis P_Proper.
Level 0: Backtrace:

P is used in hypothesis P_Proper.
Level 0: Backtrace:

P is used in hypothesis P_Proper.
Level 0: Backtrace:

P is used in hypothesis P_Proper.
Level 1: Backtrace:

P is used in hypothesis P_Proper.
Level 0: Backtrace:

P is used in hypothesis P_Proper.
Level 2: Backtrace:

Not convertible.
Level 2: Backtrace:

Not convertible.
Level 2: Backtrace:

Not convertible.
Level 1: Backtrace:

Not convertible.