Goal:
    A : Type  B : Type  C : Type  R : A → B → C → Type
  ============================
   (let H := CoreTactics.block in All3_nil = All3_nil)


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