tr>
    <td class="note_box"><img src="theme/note.gif"> In order to use spirit's
    debugging support you must ensure that appropriate overloads of 
    <tt><span class="identifier">operator</span><span class="special">&lt;&lt;</span></tt>
    taking <tt>BOOST_SPIRIT_DEBUG_OUT</tt> as its left operand are available.
    The expected semantics are those of the standard output operator.<br>
    <br>
    These overloads may be provided either within the namespace where the
    corresponding class is declared 