Comment 2 for bug 659883

James, thank you for the detailed explanation. It makes a lot of
sense. I didn't consider that the interface's primary purpose is for
configuring a security proxy.

This bug report was inspired by wanting to use union() but hitting
security proxies, so perhaps that lends weight to adding the
difference/intersection/union methods to the interface, or adding
another interface (IResultSetSetOperations, heh) for those that want
to configure those operations separately.

If nothing else I think it's worth adding a comment (or the whole
rationale) to IResultSet. I'll prepare a short branch to do that.