iex(1)> f = fn
...(1)> (_, 0, acc) -> acc
...(1)> (g, num, acc) -> g.(g, num-1, acc+1)
...(1)> end
#Function<18.128620087/3 in :erl_eval.expr/5>
iex(2)>
nil
iex(3)> f.(f,10,10)
20
iex(4)>
Functions are values …
