@spec to(non_neg_integer, [non_neg_integer]) :: non_neg_integer
  def to(limit, [h | t] = factors) do
    factors
    |> Enum.map(&make_list(limit, &1, &1, []))
    |> List.flatten
    |> &calculate_sum()
  end

  def calculate_sum([]), do: 0
  def calculate_sum(list) do
    list
    |> Enum.uniq
    |> Enum.reduce(&+/2)
  end