/prog/ challenge

Write a macro that, given a term of code, transforms all the calls to the function FOO in that term, and only those calls, into calls to function BAR with the same arguments.
Language: Common Lisp or Scheme.

sed 's/FOO/BAR/'

>>1,4
http://weitz.de/cl-ppcre/
http://www.cliki.net/regular%20expression

Yeah, I left out the ``only'' part. Does lisp require whitespace?
sed 's/\(FOO /\(BAR /'