Suppress "Part n." in <part> title/toc entry?

[Paul DuBois <paul@kitebird.com>]

I guess I should be able to figure this out from the material in Bob's book,
but I have tried and failed, so I will ask here.

I have a document that has major divisions within <part> elements. When
these are formatted for PDF output, the titles appear like this in the
document and in the TOC:

Part I. Title of first part
Part II. Title of second part
Part III. Title of third part
Part IV. Title of fourth part
etc.

I would like to suppress the "Part n." portion of these titles.  Is there a
way to do this?  Thanks.