title: How to count the number of set in the powerset?
body: <p>Given a list of integers, is there an algorithm to count the number of sets in the powerset? This shouldn't include the empty set, and, for example, <code>{1,2,3}</code> is the same as <code>{2,3,1}</code>, so they shouldn't be counted twice (i.e. powerset).</p>
  
  <p><strong>Note: The elements of the list are not necessarily unique.</strong></p>