 definitions. To accommodate for the possible different
            token <span class="emphasis"><em>value</em></span> types exposed by the various token types
            (token definitions), the general type of the token value is a <a href="http://www.boost.org/doc/html/variant.html" target="_top">Boost.Variant</a>.
            At a minimum (for the default configuration) this token value variant
            will be configured to always hold a <a href="../../../../../../../../libs/range/doc/html/range/ref