Finding source file for functions

[Andy_Trice@mtsg.ubc.ca]

Is there a function for locating the source file in which a function was 
defined?  When I was working in the Sun environment, there was a function
called *get-source-file* that did the job, but I can't find an equivalent
function in 1.3.2.  Right now I'm building my own index each time I load
the files, but this seems inefficient.