span> <span class="identifier">element_type</span> <span class="special">&amp;</span> value_pair<span class="special">)</span><span class="special">;</span></pre>
<p>If the <code class="computeroutput">value_pair's</code> key value is in the <code class="computeroutput"><a class="link" href="map.html" title="Class template map">map</a></code>, it's data value is subtraced from the data value stored in the <code class="computeroutput"><a class="link" href="map.html" title="Class template map">map</a></code>.