Problem 4: Substitute (100 pts)

Write a procedure substitute that takes three arguments: a list s, an old word, and a new word. It returns a list with the elements of s, but with every occurrence of old replaced by new, even within sub-lists.

Hint1: The built-in pair? predicate returns True if its argument is a cons pair.

Hint2: The = operator will only let you compare numbers, but using equal? or eq? will let you compare symbols as well as numbers. For more information, check out the SICP Scheme Built-In Procedure Manual.

(define (substitute s old new)
  'YOUR-CODE-HERE
)

;;; Tests
; scm> (substitute '(a (a b c) d) 'b 'e)
; (a (a e c) d)