elixir question

gistfile1.txt

# how does the handle_cast :push change the stack variable?
defmodule Stacker.Server do
  use GenServer.Behaviour

  def init(stack) do
    { :ok, stack }
  end

  def handle_call(:pop, _from, [h|stack]) do
    { :reply, h, stack }
  end

  def handle_cast({ :push, new }, stack) do
    { :noreply, [new|stack] }
  end
end