def fibonnaci(n) do
  when n == 0, do: 0
  when n == 1, do: 1
  when n > 1 do
    fibonnaci(n - 2) + fibonnaci(n - 1)
  end
end

def fibonnaci(0), do: 0
def fibonnaci(1), do: 1
def fibonnaci(n), do: fibonnaci(n - 2) + fibonnaci(n - 1)

def fibonnaci(n) do
  case n do
    0 -> ...