Goal:
  
  A : Type
  P : nat → A → Type
  Q : nat → A → Type
  n : nat
  hd : A
  tl : list A
  p : P n hd
  X : Alli P (S n) tl
  IHX : Alli (λ (n : nat) (x : A), P n x → Q n x) (S n) tl → Alli Q (S n) tl
  X0 : Alli (λ (n : nat) (x : A), P n x → Q n x) n (hd :: tl)
  hd0 : A
  tl0 : list A
  X1 : P n hd → Q n hd
  X2 : Alli (λ (n : nat) (x : A), P n x → Q n x) (S n) tl
  H0 : hd0 = hd
  H1 : tl0 = tl
  ============================
   (Q n hd)


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 := hd0 = 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.