word">proc</span> <a href="#%3Dsink%2COnce%2COnce"><span class="Identifier">`=sink`</span></a><span class="Other">(</span><span class="Identifier">dest</span><span class="Other">:</span> <span class="Keyword">var</span> <a href="once.html#Once"><span class="Identifier">Once</span></a><span class="Other">;</span> <span class="Identifier">source</span><span class="Other">:</span> <a href="once.html#Once"><span class="Identifier">Once</span></a><span class="Other">)</span> {.<span class="Identifier">error</spa