title: Getting keys from a Lookup
body: <p>How do I get the collection of keys from a Lookup&lt;> I created through the .ToLookup() method?</p>
  
  <p>I have a lookup which maps int-values to groups of instances of a custom class. I need a collection of all the int keys that the lookup contains. Any way to do this, or do I have to collect and save them separately?</p>