/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.

>>1
(defmacro foo->bar (&body body)
`(macrolet ((foo (&rest rest)
`(bar ,@rest)))
,@body))

Not sure why this is considered hard.