title: Add an element to a const set
body: <p>How would you add an element to an existing <code>Set</code> when defining it? For example, if I have:</p>
  <pre><code>const s1 = Set([1,2,3]);
  const elem = 4;
  const s2 = s1 + elem; // ?
  </code></pre>
  <p>Is using the spread operator (<code>...</code>) the correct way to do this?</p>
  <pre><code>s2 = new Set([...s, elem]);
  </code></pre>
  <p>Or is there another/better approach?</p>