How to define non-tree relationship?

[Schlaefer]

How can one define the relationship between C and A?

http://img.skitch.com/20080910-fnmgp3q5nwj8fkibg4bmfnk9ab.png

[Joel]

Sadly at the moment you can only do so in the diagram itself, the outline editor doesn't support this kind of thing (although we're thinking of ways to handle it).