title: Unique pair collection
body: <p>I want to store two strings in a collection but the combination should be unique eg</p>
  
  <pre><code>'1','2'
  '2','1'
  '1','2' -&gt; not allowed
  '2','3'
  </code></pre>
  
  <p>Which collection should I use if I want both strings to be keys? </p>