NK_TOK is not added as a key in Word-to-Index map, see </span></div>
<div class="line"><a name="l00038"></a><span class="lineno">   38</span>&#160;                <span class="comment">// contains() method below</span></div>
<div class="line"><a name="l00039"></a><span class="lineno">   39</span>&#160;                ind_to_word_[UNK_IND] = UNK_TOK;</div>
<div class="line"><a name="l00040"></a><span class="lineno">   40</span>&#160;        }</div>
<div class="line"><a name="l00041"></a><span class="lineno">