Goal:
  
  A : Type
  B : Type
  C : Type
  P : A → B → Type
  Q : B → C → Type
  R : A → C → Type
  ============================
   (let H := CoreTactics.block in
    (∀ (x : A) (y : B) (z : C), P x y → Q y z → R x z) → All2 R [] [])


DEBUG:Going to execute:
match goal with
           | |- let _ := CoreTactics.block in _ => intros _
           | |- _ => try (intro; CoreTactics.intros_until_block)
           end