Goal:
  
  A : Type
  B : Type
  P : A → Type
  Q : A → B → Type
  x : A
  l : list A
  p : P x
  H1 : All P l
  H2 : ∀ (x : A) (y : B), In y [] → P x → Q x y
  Hl : #|[]| = #|x :: l|
  IHAll : ∀ l' : list B,
            (∀ (x : A) (y : B), In y l' → P x → Q x y)
            → #|l'| = #|l| → All2 Q l l'
  ============================
   (let H := CoreTactics.block in All2 Q (x :: l) [])


DEBUG:Going to execute:
fun tac => lazymatch goal with
                      | H:_ |- _ => tac H
                      end