How to remove white space at the end of each line while saveing /only for ft=roff/ ?

Hi,

The subject nearly says it all:

When editing nroff or troff files white space at the end of a line
or on a line alone can be poisonous... :)
On other filetypes it is not neccessarily the case and harmless.

So I want to remove those white space only when saveing files in
roff/nroff/troff format.

What I accomplish so far is to remove white space for any kind
of file...but that's not exactly what I want.

How can I limit it to [nt]*roff filles only!

Cheers!
mcc


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