Does anybody know where I can find documentation on how to write annotation schemas for Callisto? I'm looking to write something a little more complicated than I can generate from a DTD -- that only gives me the ability to tag different kinds of text mentions. I'm looking to create a schema that represents a single type of relationship between five or six different kinds of textual mentions (and some of these types of mentions have attributes that I need to assign values to), and possibly having a second type of relationship between the first two instances of the first type of relationship.

(Alternatively, does anybody know of any software that would be better for this kind of schema? I've been looking at WordFreak, but it's a little clumsy, and it doesn't support attributes on its textual mentions.)