How can I replace all function calls to `f` with function calls to `g` using a racket macro? I'm new to racket and I don't know how to process syntax objects, but I believe the use case I have in mind is something a racket macro could do. Consider the following example, where I want to replace `plus` with `mul`. The macro `replace-plus-with-mul` just returns `current-seconds` as a placeholder because I don't how what to do with the syntax object to replace `plus` with `mul`. Can a macro do this?

```
#lang racket

(define-syntax replace-plus-with-mul
  (lambda (stx) #'(current-seconds)))

(define plus (lambda (x y) (+ x y)))
(define mul (lambda (x y) (* x y)))

(define a 4)
(define b 2)
(define c (plus a b))
(replace-plus-with-mul d c) ;; (define d (mul a b))
(print d) ;; should print 8

```