_REGISTER_ macro
      to associate our subtype's v-type with our subtype identifiers, we will have
      problems if someone else has also defined an identifier subtype using the same
      identifiers as we use since we will be redefining the same object-like macro
      name with a different expansion. Even if someone else has registered/pre-detected
      an identifier we are using for out subtype without defining a subtype based
      on that identifier we will be causing a problem defining our subtype