ass="mord mathnormal" style="">s</span><span class="mpunct" style="">,</span><span class="mspace" style="margin-right:0.16666666666666666em;"></span><span class="mord mathnormal" style="">a</span><span class="mclose" style="">)</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin" style="">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mord" style=""><span class="mopen nulldelimiter"></span><span class="mfrac"><span cla