title: How do I merge two dictionaries in a single expression?
body: <p>I want to merge two dictionaries into a new dictionary.</p>
  <pre><code>x = {'a': 1, 'b': 2}
  y = {'b': 3, 'c': 4}
  z = merge(x, y)
  
  &gt;&gt;&gt; z
  {'a': 1, 'b': 3, 'c': 4}
  </code></pre>
  <p>Whenever a key <code>k</code> is present in both dictionaries, only the value <code>y[k]</code> should be kept.</p>