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Ch1 HW3: Cow Graze

A pasture looks like (....)
Pastures either have grass (written 'G) and/or cows (written 'COW), or other sub-pastures inside them.
The GRAZE function takes pastures with at least one cow and removes all of their grass. It leaves pastures without cows unchanged.

Assignment

Write the GRAZE function.

Extra credit: cows in a pasture without at least one grass die (are removed), leaving only the empty pasture.

Example

There are three pastures with grass in this example, two inside a pasture that only has sub-pastures.

|-------------|
|  (1)        |
| G COW COW   |
| G G G       |
|-------------|
||---|  |----||
||G G|  |G(3)||
||G G|  |COW ||
||G  |  |G G ||
||(2)|  |G G ||
||---|  |----||
|-------------|

Written in Lisp:

(define 
  pasture-ex
    '((G COW COW G G G)
      ((G G G G G)
       (G G G COW))))

Tests

Cow is C, grass is G.

(define-test test-graze
  "Make sure the grazing works."
  (assert-equal '(C C) (graze '(G C C G G)))
  (assert-equal '((G G G G) (C C))
                (graze '((G G G G) (G C G G C))))
  (assert-equal '((C C)
                  ((G G G G)
                   (C C)))
                (graze '((G C C G G)
                        ((G G G G)
                         (G C C G G))))))