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


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 3: evaluation returns
<coq-core.plugins.tauto::is_unit_or_eq@0>
where
tauto_flags := 
of type
tauto_flags
X1 := E.t
of type
constr of type tacvalue

Level 3: Backtrace:

Tactic failure.
Level 3: Backtrace:

Tactic failure.
Level 2: Backtrace:

Tactic failure.
Level 2: Backtrace:

Tactic failure.
Level 2: Backtrace:

Tactic failure.