special">&lt;</span> <span class="identifier">dataset1_iter</span> <span class="special">&gt;</span><span class="special">(</span><span class="special">)</span><span class="special">,</span><span class="special">*</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">declval</span><span class="special">&lt;</span> <span class="identifier">dataset2_iter</span> <span class="special">&gt;</span><span class="special">(</span><span class="special">)</span><span class="s