pan class="k">var</span> <span class="nv">differenceIdentifier</span><span class="p">:</span> <span class="k">Self</span> <span class="p">{</span>
    <span class="k">return</span> <span class="k">self</span>
<span class="p">}</span>

<span class="c1">// If `Self` conforming to `Equatable`.</span>
<span class="kd">func</span> <span class="nf">isContentEqual</span><span class="p">(</span><span class="n">to</span> <span class="nv">source</span><span class="p">:</span> <span class="k">Self</span><span class="p