This is an item for the suggestion box.
XOTcl uses a number of collections: filters, mixins, children, etc. It may perhaps make sense to introduce a real collection class or abstraction, and implement those concepts directly *as* collections. Reading through a lot of XOTcl code--eg, xoRBAC--you can see that the same themes appear over and over again, such as checking to see if an object is already mixed in. Right now "append"--and only "append"--is built into the list of supplied Object methods (eg, "filterappend"). Rather than flooding Object with more and more collection management procs it might be better to expose the filter collection as an object that supports operations directly (like "append" and "contains" and "remove").
This is more like a 2.0 change--and presupposes that there will be a 2.0. :-)