title: Add / Remove key-value pairs from a Map
body: <p>How to add (and remove) key-value pairs in an Elixir map? This does not work:</p>
  
  <pre class="lang-rb prettyprint-override"><code>map = %{a: 1, b: 2, c: 3}
  
  map[:d] = 4
  </code></pre>