[Groff] .rd request

[pierrejean . f]

Helle alls,

I am trying to use the .rd request but all my efforts were in vain.
I've tried, without result, the example given here:
http://www.gnu.org/software/groff/manual/html_node/I_002fO.html#I_002fO
(I've also tried without .nx and .ex requests)

I've tried several commands, with or without U and s arguments:
cat bar | groff foo >foo.ps
groff foo >foo.ps <bar
groff foo <bar >foo.ps

Could someone give me another short example or some explanations?

Thanks a lot!

Pierre-Jean.