[plt-scheme] question about unexpected behavior using with-steps in Slideshow

I'm using with-steps, and I need to cycle through three different 
alternatives for a pict (empty, one paragraph, another paragraph).  The 
first paragraph occupies two lines but the second paragraph only takes 
one.  When I change slides and go from one paragraph to another, the 
picts shift position upwards, rather than staying where they are.  I 
would think that the bounding box would be the maximum.  I'm using the 
following form:

   ((vafter spanner) (before spanner2 (page-para "Delete some edges of
   the input graph while approximately pre\serving the optimal value.")
   (page-para "more complicated")))

"vafter" uses the bounding box of the pict, but "before" does not use 
the maximum bounding box of its two arguments.  Is that the intended 
behavior?

Thanks.  Please send responses to me since I'm not on the list.

See attached slideshow.

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