ass="line"><a name="l00043"></a><span class="lineno">   43</span>&#160;        <span class="keywordtype">size_t</span> operator()(<span class="keywordtype">size_t</span> particle_index)<span class="keyword"> const </span>{ <span class="keywordflow">return</span> particle_index; };</div><div class="line"><a name="l00044"></a><span class="lineno">   44</span>&#160;    };</div><div class="line"><a name="l00045"></a><span class="lineno">   45</span>&#160;</div><div class="line"><a name="l00047"></a><span class=