defmodule Hello.Elixir do
  def span(from, to) when from > to, do: []
  def span(from, to), do: [from | span(from + 1, to)]

  def primes_up_to(n) do
    range = span(2, n)
    range -- for a <- range, b <- range, a <= b, a * b <= n, do: a * b
  end
end```