204   <span class="keyword">virtual</span> <span class="keywordtype">size_t</span> size() <span class="keyword">const</span> = 0;
<a name="l00205"></a>00205 
<a name="l00206"></a>00206   <span class="comment">// return size of column</span>
<a name="l00207"></a>00207   <span class="keyword">virtual</span> <span class="keywordtype">size_t</span> xsize() <span class="keyword">const</span> = 0;
<a name="l00208"></a>00208 
<a name="l00209"></a>00209   <span class="comment">// return size of features</span>
<a n