 because
      VMD macros which generically return the type of a sequence or sequence element
      will return our subtype as the type rather than just BOOST_VMD_TYPE_IDENTIFIER
      which some programmer might expect.
    </p>
<p>
      The gist of this is that if we define a user-defined subtype its identifiers
      need to be unique within a given translation unit, and yet unique names make
      it harder for an end-user to use macros more naturally. In our given example
      with the mythical udef 