Guided Practice 5.1: Generalizing Pizza

Download replace-all-anchovies-with-onions.rkt and write its generalizations replace-all-anchovies and replace-topping, given by the following contracts and purpose statements:

;; replace-all-anchovies-with-onions 
;;   : Pizza -> Pizza
;; RETURNS: a pizza like the given pizza, but with
;; anchovies in place of each layer of onions

;; replace-all-anchovies : Pizza Topping -> Pizza
;; RETURNS: a pizza like the given pizza, but with 
;;  all anchovies replaced by the given topping.

;; replace-topping : Pizza Topping Topping -> Pizza
;; RETURNS: a pizza like the given one, but with 
;; all instances of the first topping replaced by
;; the second one.

[ANSWER]