[Groff] strike through fonts?

[Larry McVoy]

Hi, I'm using groff to work on legal docs with lawyers and they want 
old/new markups where they can see what the old stuff was but it is
struck through like Word does.

I've developed formatting tools which take troff input and reformat
things such that each sentence starts on a new line and is formatted
to the same length where possible.  So I can format the new/old input
and diff that to get sentence based diffs.  For the strike through
stuff I'll make a version which formats on word boundaries so we get
word based diffs.

My thought was to post process the diff output, eating both the old
and new and use a different font for the old, a font which has all 
the characters with a horizontal line through the width of the char.

Do you see what I am trying to do?  Is there a better way to do it?

Maybe this is a more clear way to ask the question:

    Suppose I had a sequence of words in the form

        unchanged
        unchanged
        old | new
        unchanged
        unchanged
        old | new
        unchanged

    How do I make troff take that input and produce the "old" with a
    horizontal line through it and "new" underlined?