Huh, works for both @callback and def. And since the implementing module is its own namespace of course, the import Kernel, except: [...] is not needed there.

defmodule Filter do
  @callback not(term) :: String.t()
  @callback term or term :: String.t()
  @callback unquote(:in)(term, term) :: String.t()
end

defmodule Impl do
  @behaviour Filter

  @impl Filter
  def not(a), do: "!#{a}"

  @impl Filter
  def a or b, do: "#{a} or #{b}"

  @impl Filter
  def unquote(:in)(a, b), do: "#{a} in #{inspect(b)}"
end

Impl.not(1) <> ", " <> Impl.or(1, 2) <> ", " <> Impl.in(3, [3, 4])

#=> "!1, 1 or 2, 3 in [3, 4]"

Soo, yeah… thanks for the TIL!