Making owner & requires mutually exclusive is not a good idea.
... At the moment, I kind of like the idea
of saying that owner & requires are mutually exclusive (i.e. an object
is either a container or contained, but not both), and that the order of
hidden objects and their owners should be specified, with the owner
coming last. But we should recognize the limitations we're creating if
we do this....
A severe limitation. We should permit nesting and, as
you say, treat cycles as errors. Throughout this discussion I have
been thinking in terms of documents that reference object over which
the sender has no control but which may in turn reference other
objects. I expect the MUA to handle the incorporation of the the
referenced parts (the ojects could be in MIME representation).
Consequently, I see nesting as critical.
Here's a less abstract statement of the problem. I want to send you an
anthology with chapters written by many different people. Each
chapter is a MIME message with contained elements. My MIME message
needs to contain each chapter, hence each chapter is both contained
and a container.