invoked a user-defined marker type had been specified,
      then the second optional parameter is that marker type, else it is not necessary
      to specify the optional second template parameter. Since the marker type is
      saved as the nested type boost::tti::marker_type once we invoke the metafunction
      generated by BOOST_TTI_MEMBER_TYPE we can always use that as our second template
      parameter to 'boost::tti::valid_member_type' if we like.
    </p>
<p>
      The second metafunction, boost::