title: How to get union of several immutable.js Lists
body: <p>So, I have List a:</p>
  
  <pre><code>let a = Immutable.List([1])
  </code></pre>
  
  <p>and List b:</p>
  
  <pre><code>let b = Immutable.List([2, 3])
  </code></pre>
  
  <p>I want to get List <code>union === List([1, 2, 3])</code> from them. </p>
  
  <p>I try to <a href="http://facebook.github.io/immutable-js/docs/#/List/merge" rel="noreferrer">merge</a> them fist:</p>
  
  <pre><code>let union = a.merge(b); // List([2, 3])
  </code></pre>
  
  <p>It seems like <code>merge</code> method operates with indexes, not with values so overrides first item of <code>List a</code> with first item of <code>List b</code>. So, my question is what is the most simple way to get union of several lists (ideally without iterating over them and other extra operations).</p>