Custom hyphenation files

I was tring to follow the instructions for manually installing OFFO
hyphenation files.
 https://xmlgraphics.apache.org/fop/1.0/hyphenation.html shows
how to use the source files (not the compiled fop-hyph.jar). I did the
following:

1) I renamed $FOP_HOME/lib/fop-hyph.jar.
2) I copied en_GB.xml from offo-hyphenation.zip to /path/to/book.xml
3) Added the following entry to fop.xconf:
  <hyphenation-pattern lang="en">en_GB</hyphenation-pattern>
4) Then
  cd /path/to/book.xml
  xsltproc -o book.fo xsl-ns/fo/docbook.xsl book.xml
  fop -c fop.xconf book.fo book.pdf

The result was:
INFO: Rendered page #1.
Exception in thread "main" java.lang.StackOverflowError
        at org.apache.fop.hyphenation.TernaryTree.insert(TernaryTree.java:191)
        at org.apache.fop.hyphenation.TernaryTree.insert(TernaryTree.java:244)
...    [line above repeated several times]

        at org.apache.fop.hyphenation.TernaryTree.insert(TernaryTree.java:244)

Two questions:

1) What am I doing wrong? By the way, it is fop-svn 1728886.

2) How do you choose the location of en_GB.xml? I tried setting
hyphenation-base. https://xmlgraphics.apache.org/fop/2.0/configuration.html
says that relative URIs for hyphenation-base are evaluated relative to the base
URI of the configuration file. Does that mean that mean that hyphenation-base
would have to be set to ../<back-to-root>/full/path/to/en_GB-location?

For what it is worth, I set <hyphenation-base>.</hyphenation-base>.

Regards,
John Brown.