span> <span class="special">&gt;</span> <span class="special">&amp;</span> a<span class="special">,</span> 
                     <span class="keyword">const</span> <a class="link" href="../boost/histogram/histogram.html" title="Class template histogram">histogram</a><span class="special">&lt;</span> <span class="identifier">A2</span><span class="special">,</span> <span class="identifier">S2</span> <span class="special">&gt;</span> <span class="special">&amp;</span> b<span class="special">)</span><span class