indexterm in footnote

[maxwell <maxwell@umiacs.umd.edu>]

In the description of footnotes in the DocBook 5 documentation
   http://www.docbook.org/tdg5/en/html/footnote.html
under the section "Additional Constraints" it lists the following
constraint:
   indexterm must not occur in the descendants of footnote
I'm confused about how indexterms are supposed to be used in footnotes. 
Typically (I think) you have a para inside the footnote (at least that's
how the example works on that same page).  The implication of the above
constraint would then be that any indexterm in a footnote has to be outside
the text of the footnote, I guess like this:

   <para>An annual percentage rate<footnote>
   <indexterm>...prime rate...</indexterm>
   <para>The prime rate, as published in the <citetitle>Wall Street
   Journal</citetitle> on the first business day of the month,
   plus 7.0%.
   </para>
   </footnote>
   ...

Is that the intention?  One perhaps undesirable result of this would be
that if the footnote extends over two pages, the indexterm might point to
the wrong page.  Or is the expectation that in the output document, the
index would say something like "prime rate: 23 footnote 7"?

(Of course, one might say that footnotes should never extend over more than
one page.  That's a rule we academics tend to break...)

   Mike Maxwell