How to represent many-to-many relation?

[Mark-Jason Dominus <mjd@xxxxxxxxxx>]

I'm going to get a contract to write a book, but I don't know yet what
book it is.  There are six or seven possibilities open.
Each possible book has some overlap with the others.

I also have ideas for chapters, each of which has a topic.   Some
chapters are appropriate for several books.

For example, let's suppose that Book A will contains chapters on
topics X and Y; book B will contain chapters on topics X and Z; and
book C will contain chapters on all three topics.

I can't use d.contents and d.inside because if I do, no topic can
be in the contents of more than one book:

        +--> d.inside
        |
        V d.contents

        Book A --- Topic X
                   |
                   Topic Y 

	Book B --- Topic X
                   |
                   Topic Z

This is illegal, because here Topic X has two links in the -d.inside
direction, and that is Not Allowed.

What's a good way to represent this idea in Zigzag?  

Mark-Jason Dominus                           	               mjd@xxxxxxxxxx