Yeah if you are making an is_* guard then you can only use other guards inside it. Take a look at my defguard library to see how I generate guards.
(EDIT: Do note, the entire code of that library is a huge hack that overwrites def and defp and so forth and should not be used as an example of what to do except for the guard part itself, that library is purely an example to see ‘if I could do something’, not ‘should’ ^.^)
