added
            to the lexer at any time, usually in the constructor of the user defined
            lexer class:
<pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Lexer</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">example_tokens</span> <span class="special">:</span> <span class="identifier">lex</span><span class="special">::</span><span cl