title: get list of all nondecreasing sets of list in haskell
body: <p>How can I generate all the possible nondecreasing sets of the elements of a list with current length?</p>
  
  <pre><code>getSets :: [Int] -&gt; Int -&gt; [[Int]]
  ...
  
  &gt; getSets [0..9] 3
  [[0,0,0],[0,0,1]..[3,9,9],[4,4,4]..[8,9,9],[9,9,9]]
  </code></pre>