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


DEBUG:Going to execute:
fun id =>
             CoreTactics.on_last_hyp
              ltac:(fun id' =>
                      match id' with
                      | id => idtac
                      | _ => revert id'; CoreTactics.revert_until id
                      end)