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
  ============================
   ((t = t) + P t t)%type


DEBUG:Going to execute:
<Coq.Init.Tauto.is_disj> flags X1; elim id; intro; clear id
Level 2: evaluation returns
<coq-core.plugins.tauto::is_disj@0>
where
tauto_flags := 
of type
tauto_flags
X1 := P hd hd'
of type
constr of type tacvalue

Level 2: Backtrace:

Tactic failure.
Level 2: Backtrace:

Tactic failure.
Level 1: Backtrace:

Tactic failure.
Level 1: Backtrace:

Tactic failure.
Level 1: Backtrace:

Tactic failure.
Level 1: Backtrace:

Tactic failure.