title: Define: What is a HashSet?
body: <p><strong>HashSet</strong>
  The C# HashSet data structure was introduced in the .NET Framework 3.5. A full list of the implemented members can be found at the <a href="http://msdn.microsoft.com/en-us/library/bb359438.aspx" rel="noreferrer">HashSet MSDN</a> page.</p>
  
  <ol start="2">
  <li>Where is it used?</li>
  <li>Why would you want to use it?</li>
  </ol>