     will be converted to the requested token type (parser attribute) exactly
              once. This happens at the time of the first access to the token value
              requiring the corresponding type conversion. The converted token value
              will be stored in the <a href="http://www.boost.org/doc/html/variant.html" target="_top">Boost.Variant</a>
              replacing the initially stored iterator range. This avoids having to
              convert the input sequence to the token value m