258"></a>00258 
<a name="l00259"></a>00259   <span class="comment">// return backward log-prob of the j-th tag at i-th token</span>
<a name="l00260"></a>00260   <span class="keyword">virtual</span> <span class="keywordtype">double</span> beta(<span class="keywordtype">size_t</span> i, <span class="keywordtype">size_t</span> j) <span class="keyword">const</span> = 0;
<a name="l00261"></a>00261 
<a name="l00262"></a>00262   <span class="comment">// return emission cost of the j-th tag at i-th token</span>
<a 