FO: Vertical alignment in <fo:flow>

I wonder what the principles are for vertical alignment in <fo:flow>. I'm
making presentation slides and would like the content, which is often just
a couple of words per slide, center aligned both horizontally and
vertically. My first idea was something like this:

<fo:flow>
  <fo:block-container
    text-align="center"
    display-align="center"
    height="170mm">
    ...
  </fo:block-container>
</fo:flow>

But neither height nor display-align seem to makes any difference here. Is
this what absolute-position="fixed" is meant for? Can anyone give an
example of which properties are involved?

Thanks in advance,

Gustaf


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