ation
      unit. This is the main difference between just defining identifiers and defining
      an identifier subtype.
    </p>
<p>
      Recall that when we just register and pre-detect identifiers we will have no
      problems if the same identifiers already have been registered and pre-detected
      within the same translation unit. This is because we are just redefining the
      exact same macro if this is the case.
    </p>
<p>
      But with identifier subtypes, when we use the BOOST_VMD_SUBTYPE