Goal:
  
  A : Type
  B : Type
  n : nat
  t : A
  P : B → A → A → Type
  b : B
  bs : list B
  hd : A
  hd' : A
  tl : list A
  p : P b hd hd'
  e : #|bs| = #|tl|
  hnth : nth_error tl n = Some t
  ============================
   (nth_error tl n = Some t
    × (t = t) + (∑ i' : B, nth_error bs n = Some i' × P i' t t))


DEBUG:Going to execute:
Ltac.split
Level 2: evaluation returns
<coq-core.plugins.tauto::is_conj@0>
where
tauto_flags := 
of type
tauto_flags
X1 := nth_error tl n = Some t
      × (t = t) + (∑ i' : B, nth_error bs n = Some i' × P i' t t)
of type
constr of type tacvalue