[OS X TeX] Underbracket woes

[Jeff Sherman]

Greetings all,
I'm trying to create brackets that connect terms in an equation much 
like \underbrace, but straight instead of curly.  They should also be 
nestable, so I could connect two inner terms while connection two outer 
terms with a bigger bracket. (Physicists, I'm trying to make the 
contraction operator used in quantum field theory, see Peskin & 
Schroeder p. 89 for an example though they use overbrackets, which are 
fine too).

Such a device is defined at 
http://www.lyx.org/help/underbracket/bracket.php3 (and for brevity I 
won't paste it here) but when I try something simple, like 
\underbracket{AB}, I get the error message:

! Use of \@ doesn't match its definition.
\underbracket ->\@i
                    fnextchar [ {\@underbracket}{\@underbracket 
[\@brackethei...
l.903 \underbracket
                    {AB}

I'm nowhere near competent enough in pure TeX to understand or fix the 
problem, and I'm hoping someone wouldn't mind the, erm, uh, challenge.  
Alternatively, is there a better way to do what I'm trying to do?  The 
aforementioned textbook was clearly done in TeX, so there must be a way 
-- perhaps I should bother the authors!  =:-)

Thanks,
-Jeff Sherman


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