so this compiled. interesting...

default_args_fun.exs

  def concurrent_factorial(n) when is_integer(n) and n > 0 do
    concurrent_factorial(1..n)
  end
  def concurrent_factorial(range, worker_pool_by_nodenames \\ worker_pool_by_nodenames)
  def concurrent_factorial(%Range{first: start, last: finish}, worker_pool_by_nodenames) do
    pmap(split_range_of_numbers(start..finish), fn(range) -> range |> Enum.reduce(&(&1*&2)) end, worker_pool_by_nodenames) |>
    # one more reduction...
    Enum.reduce(&(&1*&2))
  end