lper function allowing to create a distinct() construct from </span>
<span class="comment">// an arbitrary tail parser</span>
<span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Tail</span><span class="special">&gt;</span>
<span class="keyword">inline</span> <span class="keyword">typename</span> <span class="identifier">traits</span><span class="special">::</span><span class="identifier">distinct_spec</span><span class="specia