an><span class="w"> </span><span class="n">i</span><span class="o">+=</span><span class="n">step</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w">  </span><span class="n">callable</span><span class="p">(</span><span class="n">i</span><span class="p">);</span>
<span class="p">}</span>
<span class="c1">// negative step: first, first-step, first-2*step, ...</span>
<span class="k">for</span><span class="p">(</span><span class="k">auto</span><span class="w"> </span><