Define a custom guard using Macro

Hi, i’m currently writing version 2 of Mizur.

Now, using the Mizur module in another module generates the entire API in the receiver module. This generally avoids dependence on two modules in use.
The Mizur.System module exposes several macros, including the macro type name = ... that creates several functions in the receiver module.

I would like to take advantage of this moment to create a specific guard.
Currently I have this code:

defmodule Mizur.System do
   defmacro __using__(_opts) do 
    quote do 
      import Mizur.System
      # here some uncontextual code ! 
      # This code is included into the macro "type name = ..."
      is = String.to_atom("is_" <> Atom.to_string(name))
      defmacro unquote(is)(e) do
        mod = __MODULE__
        typ = Module.concat(mod, Type)
        n = unquote(name)
        quote do
          case unquote(e) do
            %mod{type: %typ{name: unquote(n)}} -> true
            _ -> false
          end
        end
      end

      # .. the rest of the code, uncontextual to ! 

If I use the code in a simple expression, it will work, but if I write somethings like that :

test "for macros" do 
    import Length
    c = cm(12)

    IO.inspect is_cm(c) # This code is valid.

    case c do 
      x when is_cm(x) -> IO.inspect "lol"
      _ -> IO.inspect "snif"
    end 

I have this output :

** (CompileError) test/mizur_test.exs:112: invalid expression in guard
    expanding macro: MizurTest.Length.is_cm/1
    test/mizur_test.exs:112: MizurTest."test for macros"/1
    (elixir) lib/code.ex:370: Code.require_file/2
    (elixir) lib/kernel/parallel_require.ex:57: anonymous fn/2 in Kernel.ParallelRequire.spawn_requires/5

I do not know if I made any mistake.

Regards, Xavier !