I want to be able to [doctest](https://hexdocs.pm/ex_unit/master/ExUnit.DocTest.html) the *implementation* of a [protocol](https://elixir-lang.org/getting-started/protocols.html) in Elixir.

Here is some example code:

```
defprotocol Size do
  def size(data)
end

defmodule Foo do
  defstruct []

  defimpl Size do
    @doc """

    ## Examples

        iex> Size.size(%Foo{})
        0
    """
    def size(foo) do
      0
    end
  end
end

```

This test code doesn't work (because it only finds functions defined in the `Foo` module, outside of the `defimpl`):

```
ExUnit.start()

defmodule FooTest do
  use ExUnit.Case
  doctest Foo
end

```

How can I test the `defimpl` block?