Placement of auto-generated TOC

I've got a book, where the TOC should be inserted at the end of the 
frontmatter.

I've got several preface elements in the frontmatter, so the book is 
essentially:
<book>
  <preface>...</preface>
  <preface>...</preface>
  <preface>...</preface>
  <toc/>
...
</book>

I've set the <xsl:param name="process.empty.source.toc" select="1"/> 
param in my customization layer, but the TOC keeps showing up at the 
front of the doc. What am I missing?

I'm using the 1.74.0 stylesheets.

Thanks and best regards,

-- Scott