Trying to figure out function head bindings for Elixir Macro

example.ex

defmodule Aggregate do
  defmacro replay(struct, aggregate, do: block) do
    quote bind_quoted: [struct: struct] do
      @events [struct|@events]
      def __replay__(struct, unquote(aggregate)), do: unquote(block)
    end
  end
end

defmodule User do
  defstruct [:activated?]

  replay %UserCreated{} = event, aggregate do
    %{aggregate | activated?: event.activated?}
  end
end

def User do
  def __replay__(%UserCreated{} = event, aggregate) do
    %{aggregate | activated?: event.activated?}
  end
end