title: Finding all subsets of a set (PowerSet)
body: <p>I want to find all subsets of a given set.
  I got string set defined as following: <code>HashSet&lt;String&gt; L</code> and I want to use in a loop all of its subsets as: <strong>for each <img src="https://i.stack.imgur.com/8xST7.png" alt="A subset of L"> do something</strong>.
  Is there an easy way with low complexity to do that?</p>