title: Subsets having duplicates?
body: <p>Can a subset, not a proper subset, have duplicate values?</p>
  
  <p>Example</p>
  
  <pre><code>A = {1, 2, 3, 3, 4, 5}
  B = {1, 2, 3, 4, 5}
  </code></pre>
  
  <p>is <code>AcB</code> or does the duplicate value cancel that out?</p>