title: How to define an Ordering in Scala?
body: <p>Having <code>val hm: HashMap[org.joda.time.DateTime, MyType]</code> I am trying to get the first and the last <code>DateTime</code> of the set by means of <code>hm.keys.min</code> and <code>hm.keys.max</code> respectively but the compiler says <code>No implicit Ordering defined for org.joda.time.DateTime</code>. How to define this ordering (both implicit and explicit options are interesting)?</p>