Returns the new value of the key.</p>
<p>If the key is present, invokes <code>update</code> with the current value and stores
the new value in the map.</p>
<p>If the key is not present and <code>ifAbsent</code> is provided, calls <code>ifAbsent</code>
and adds the key with the returned value to the map.</p>
<p>It's an error if the key is not present and <code>ifAbsent</code> is not provided.</p>
    </section>
    
    <section class="summary source-code" id="source">
      <h2><span>Implementation</span></