i>, <i>op</i>(<i>r</i>,
        <i>state</i>)) ... <i>macro</i>(<i>r</i>, <i>op</i>(<i>r</i>, ... <i>op</i>(<i>r</i>,
        <i>state</i>) ... )) </div>
    </div>
    <div> At certain times, it may be necessary to perform the concatenation
      with <b>BOOST_PP_CAT</b> rather than the preprocessor token-pasting
      operator.&nbsp; This happens when the <i>r</i> value is a macro
      invocation itself.&nbsp; It needs a delay to allow it to expand.&nbsp; The
      syntax in such a scenario becomes:
    