Setting html-index to true in customization of Docbook

Hi!

I am trying to use the customizations of Docbook DSSSL. Among all the
customiation, there is one I do not understand.

If I set

(define html-index
  ;; REFENTRY html-index
  ;; PURP HTML indexing?
  ;; DESC
  ;; Turns on HTML indexing.  If true, then index data will be written
  ;; to the file defined by 'html-index-filename'.  This data can be
  ;; collated and turned into a DocBook index with bin/collateindex.pl.
  ;; /DESC
  ;; AUTHOR N/A
  ;; /REFENTRY
  #t)

All I have is an empty 'HTML.index' file. How one is suppose to use it?

Thanks

By the way, if some one knows how to search the archives of this mailing
list, I would be happy to search!

Phil.