Goal:
  
  A : Type
  R1 : A → A → Type
  R2 : A → Type
  R3 : A → Type
  hd : A
  hd' : A
  tl : list A
  p : R1 hd hd'
  H : All R2 (hd :: tl)
  ============================
   (let H := CoreTactics.block in
    (∀ x : A, R2 x → R3 x)
    → (∀ x y : A, R1 x y → R2 x → R3 y) → All R3 (hd' :: tl))


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