[Groff] Underbrace

[Cedric Sodhi]

Dear GRoffers,

I just have a simple question to save me the unessary effort to
write it myself if someone has already written it:

Is there something like latex' "underbrace"/"overbrace" in groff/eqn or
has someone written a macro for it already? I really miss that feature
because it's a pretty concise way to mark and explain terms in
equations.

If not and someone happens to be in a creative mood I'd appreciate any
help because I'm not that much of a great tmac writer ^^

kind regards,
C. Sodhi