[Zope] beginner's question: references

[Perrin Harkins]

This is such a stupid question, but I can't seem to figure it out from
the docs.  How do I make an object in Zope reference another object?  I
don't mean containing like a folder, I mean one object referencing
another in a different part of the hierarchy.  For example, if I make a
class to represent movies, and another to represent theaters and I want
the movies to reference the theaters they're playing at and vice versa. 
How does Zope handle this?

Please try to frame an answer in the context of ZClasses, since that's
what I'm using.  (Or tell me if ZClasses can't do this yet.)

Thanks,
Perrin