title: Usage of HACK and UNDONE comment tags
body: <p>Many software engineers are familiar with the usage of special comment “tags” that can be added to their code comments, for use in searches, automated task tracking, and so forth.  Some of the most popular are FIXME, TODO, UNDONE, and HACK. </p>
  
  <p>I'm a bit confused with the usage of HACK and UNDONE tags. Little help please?</p>
  
  <p><em>Bonus points for showing the basic difference between FIXME and TODO</em></p>