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:
match goal with
           | |- ?X1 → ?X2 => intro
           | |- ¬ _ => unfold not at 1; intro
           end
Level 0: Backtrace:

No matching clauses for match.