Goal:
  
  A : Type
  t : A
  P : nat → A → A → Type
  i : nat
  hd' : A
  tl : list A
  p : P i t hd'
  ============================
   ((t = hd') + P i t hd')%type


DEBUG:Going to execute:
<Coq.Init.Tauto.is_empty> flags X1; elim H
Level 3: evaluation returns
<coq-core.plugins.tauto::is_unit_or_eq@0>
where
tauto_flags := 
of type
tauto_flags
X1 := ((t = hd') + P i t hd')%type
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.