Goal:
  
  A : Type
  P : A → Type
  Q : A → Type
  x : A
  l : list A
  p : P x
  Hl : All P l
  Hl' : All Q (x :: l)
  IHHl : All Q l → All (λ x : A, P x × Q x) l
  ============================
   (let H := CoreTactics.block in All (λ x : A, P x × Q x) (x :: l))


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