vertical centering of a block in XSL-FO.

I have two blocks that are beside each other in a table.  One block is
larger than
the other, and I want the smaller one centered vertically.  The number
of lines in
the smaller box will vary.  With external graphics, I see there was a
way to center
it vertically, but didn't find anything for text blocks.    The only way
that I found
to get "close" is to count the lines in the smaller section and line
them up
horizontally with lines in the other half.  Unfortunatley if I have an
odd number of
lines it won't come out correct.  I can't do subsitutions into attribute
values in the
block using xsl, so I can't really do the calculations myself and put in
a space-before.
I am currently using Apache's implementation of XSL-FO.  Any
suggestions?

                                Thanks,
                                    Fred Romelfanger
                                    fred@xxxxxxxxx



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