[Groff] Centering pictures and captions with mm

[Daniel de Kok]

Hi,

I have a fairly newbie-ish question about the mm marcos. I have some 
pictures that I would like to center. To keep the captions of the image on 
the same page, I have enclosed the picture and display in a block. The 
problem that I have is that with setting the display formatting to CB, the 
caption is included in the block, and both the picture and the caption are 
not centered. For example:

.DS CB
.PS
right
arrow
box
arrow
.PE
.FG "An example picture"
.DE

Making a display in a display solves this:

.DS
.DS CB
.PS
right
arrow
box
arrow
.PE
.DE
.FG "An example picture"
.DE

But this requires that Ds is set to 0, to prevent that the gap between the 
caption and the image becomes to huge. Is there a more elegant solution to 
this problem?

-- Daniel