Magritte: Accessors for classes with collection members

Hello all, In Ramon's blog [1] he mentions: "When creating a description for that collection, I can specify my custom #accessor: rather than using the default #selectorAccessor: which then writes to my domain object with #addUser: and #removeUser:." Looking at the code, I don't see how this is done. Isn't the write accessor going to get a complete collection? Is the idea to just diff the received collection from the one the object had and make calls to #add and #remove as appropriate, or is there a way to only be told about the new objects in the collection? Thanks, Jason [1] http://onsmalltalk.com/programming/smalltalk/using-magritte-with-seaside/