title: Join on compare keys on one side of equals (startswith)
body: <p>I´m joining two lists, one with bus paths, the other with disk paths that are bus subpaths. So basically I want to join on diskpath.startswith(buspath). The problem is Join On only allows one key on one side of equals.</p>
  
  <p>I want this:</p>
  
  <pre><code>Dictionary&lt;string, string[]&gt; disksPerBus;
  
  Dictionary&lt;string, string&gt; busesHWPath;  // key Bus Path, Value Bus name
  Dictionary&lt;string, string&gt; disksHWPath;  // Key Disk Path, Value Disk name 
  
  from busHWPath in busesHWPath 
  join diskHWPath in disksHWPath on diskHWPath.Key.StartsWith(busHWPath.Key)
  ...
  </code></pre>
  
  <p>The final goal is to get a dictionary (disksPerBus) with the bus name (busHWPath.Value), and all the disks attached to that bus (diskHWPath.Value).</p>