simple Macro.prewalk implementation

prewalk.ex

defmodule Tr do
  def tr(ast, fun) do
    case fun.(ast) do
      {x1, x2, [x3, x4]} -> {x1, x2, [tr(x3, fun), tr(x4, fun)]}
      {x1, x2, x3} -> {x1, x2, tr(x3, fun)}
      x -> x
    end
  end
end