Goal:
  
  A : Type
  n : nat
  t' : A
  P : A → A → Type
  hd : A
  hd' : A
  tl : list A
  p : P hd hd'
  H : nth_error tl n = Some t'
  ============================
   (nth_error tl n = Some t' × (t' = t') + P t' t')


DEBUG:Going to execute:
<Coq.Init.Tauto.flatten_contravariant_disj> flags X1 X2 id
Level 2: evaluation returns
<coq-core.plugins.tauto::flatten_contravariant_conj@0>
where
tauto_flags := 
of type
tauto_flags
id := P
of type
constr
X2 := A → Type
of type
constr
X1 := A
of type
constr of type tacvalue

Level 2: Backtrace:

Tactic failure.
Level 2: Backtrace:

Tactic failure.
Level 1: Backtrace:

Tactic failure.