title: How to implement the empty set - ∅?
body: <p>Assuming that you want to implement set theory concepts such as <code>element</code>, <code>set</code>, <code>collection</code> and <code>relation</code> in Java: How would you represent the <em>empty set</em> <code>∅</code>?</p>
  
  <p>Do I mislead myself, if I think of the <code>NULL</code> concept as it is used by the three-valued logic of databases?</p>