Goal:
  
  A : Type
  B : Type
  P : B → A → A → Type
  Q : A → Type
  hd : A
  tl : list A
  b : B
  bs : list B
  hd' : A
  p : P b hd hd'
  e : #|bs| = #|tl|
  X : Q hd
  X0 : All Q tl
  ============================
   (Q 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 0: Backtrace:

Q is used in conclusion.
Level 0: Backtrace:

Q is used in conclusion.
Level 0: Backtrace:

Q is used in conclusion.
Level 0: Backtrace:

Q is used in conclusion.
Level 0: Backtrace:

Q is used in conclusion.
Level 1: Backtrace:

Q is used in conclusion.
Level 0: Backtrace:

Q is used in conclusion.
Level 2: Backtrace:

Not convertible.
Level 2: Backtrace:

Not convertible.
Level 2: Backtrace:

Not convertible.
Level 1: Backtrace:

Not convertible.