[Groff] Please help on a math symbol

[Min Xu]

Hi, 

I am now facing a problem with the ensemble average structure in *roff.
An average is commonly written in a form of 
   <a> 
to denote the mean of a. However, when we have multiline structure inside, 
i.e., 
    / a \
     ---
    \ b /
does groff have a method in representing the larger version average symbol?

A mere "left <   ...    right >" does not produce the desired result.

Thanks.

Min Xu
12/5/02