nested type, and there was no nested type 'InnerType' within the enclosing
      type 'T, the compiler would give us an error immediately.
    </p>
<p>
      By using BOOST_TTI_MEMBER_TYPE we have a type to work with even when such a
      type really does not exist. Naturally if the type does not exist, the type
      which we have to work with, being a marker type, will generally not fulfill
      any other further functionality we want from it. This is good and will normally
      produce the correct res