title: What's the output of 'd = {0, 1, 2} for x in d: print(d.add(x)) .And why?
body: <p>The Python code is:</p>
  
  <pre><code>d = {0, 1, 2}
  for x in d:
      print(d.add(x))
  </code></pre>
  
  <p>What is the output, and why?</p>