defmodule Funcs do

  # Naive, slow and buggy implementation:
  def fib(0), do: 0
  def fib(1), do: 1
  def fib(n), do: fib(n-1) + fib(n-2)

  # Fast version:
  #  def fib(n) when not is_number(n) or n < 0, do: raise("arg!")
  #  def fib(n), do: fib(0, 1, n)
  #
  #  def fib(a, b, 0), do: a
  #  def fib(a, b, n), do: fib(b, a+b, n-1)
  
end