vertical centering with XSL-FO

I have two blocks on the left, with one block next to it on the right.
The blocks
on the left side of the page take up more vertical space than the one on
the right.
I want to center the text vertically in the right hand block.  I don't
know ahead of
time how much space the text will take up.  Is there a way to vertically
center a
block of text with XSL-FO?  I noticed that external graphics can be
centered
vertically, but I didn't see anything for centering blocks.  I tried
doing calculations
with the height, but XSL tells me that they aren't integer values.  I
think the closest
I can come is making the individual lines on both sides into table cells
and then count
the lines on the right and line them up horizontaly with the lines on
the left.  In my
case this won't work very well of I have an odd number of lines on the
right side.
I am currently using the apache implementation of XSL-FO which doesn't
presently
support row/column spanning, and it wasn't clear from the specs what
would happen
with the text alignment when spanning a table row.  Any suggestions?
                            Thanks,
                                   Fred Romelfanger
                                   fred@xxxxxxxxx


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