Goal:
  
  A : Type
  B : Type
  P : A → Type
  Q : A → B → Type
  H2 : ∀ (x : A) (y : B), In y [] → P x → Q x y
  ============================
   (let H := CoreTactics.block in All2 Q [] [])


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