Goal:
  
  A : Type
  P : A → A → Type
  t : A
  x : A
  l : list A
  l' : list A
  r : P x t
  Hall : All2 P l l'
  IHHall : ∀ n : nat,
             nth_error l' n = Some t
             → ∑ t' : A, nth_error l n = Some t' × P t' t
  ============================
   (P x t)


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