defmodule A do
      defstruct [...]
    end
    defmodule B do
      defstruct [...]
    end
    defprotocol C do
      def foo(a, b)
    end
    defimpl C, for: Any do
      def foo(x1 = %m1{}, x2 = %m2{})
        Module.concat(m1, m2).foo(x1, x2)
      end
    end
    defmodule A.A do
      def foo...
    end
    defmodule A.B do
      def foo...