[Seaside] Where is some definitive document on callback priorities, response priority

[Fritz Schenk]

After spending a couple of months solving the puzzle at the end of Chapter 12.2, 
I finally solved it in two ways.
1. Suggested by Joachin Small World - use #cancelButton and, rely on priority 
-5, and use `self handleAnswer: false`
2. Suggestion by Lukas Renggli to simply use a copy in the editor and replace 
the collection entry corresponding to the contact.

The problem I face is that there is little documentation on how priority is used 
and assigned in both #cancelButton and in #response.

Thanks