Goal:
  
  A : Type
  P : nat → A → Type
  Q : A → Type
  n : nat
  hd : A
  tl : list A
  p : P n hd
  X : Alli P (S n) tl
  X0 : All Q (hd :: tl)
  x : A
  l : list A
  X1 : Q hd
  X2 : All Q tl
  H0 : x = hd
  H1 : l = tl
  X3 : Alli (λ (k : nat) (x : A), P k x × Q x) (S n) tl
  ============================
   (P n hd)


DEBUG:Going to execute:
<Coq.Init.Tauto.is_unit_or_eq> flags X1; Ltac.cut X2;
            [ intro; clear id
            | Ltac.cut X1; [ exact id | constructor 1; fail ] ]
Level 2: evaluation returns
<coq-core.plugins.tauto::is_unit_or_eq@0>
where
tauto_flags := 
of type
tauto_flags
X1 := nat
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.