Problem$^a$ 3.   Here are data definitions from an environmental surveillance program for rivers:

(define-struct end (location distance in))
(define-struct joint (location distance left right))
(define-struct well (location))

;; A River (system) is a:
;; -- (make-end Symbol Number Tributary)

;; A Tributary is one of:
;; -- (make-well Symbol)
;; -- (make-joint Symbol Number Tributary Tributary)

Draw ``arrows' into the data definitions. Provide one data example for River that involves all three structures. Develop as many templates as needed for this system of data definitions.

Solution [PT 1: data example]

(make-end 'A 10 
             (make-joint 'B 20 
                               (make-well 'C) 
                               (make-well 'D)))

[PT 7: 2 for two templates, 3 for arrows, 1 for all sel, 1 for cond]

;; pro-river : River -> ???
(define (pro-river r)
  ... (end-location r) ... (end-distance r) 
  ... (pro-tributary (end-in r)) ...)

;; pro-tributary : Tributary -> ??? 
(define (pro-tributary t)
  (cond
    [(well? t) ... (well-location t) ...]
    [else ... (joint-location t) ...
          ... (joint-distance t) ...
          ... (pro-tributary (joint-left t)) ...
          ... (pro-tributary (joint-right t)) ...]))