1 (defun make-mobile (left right)
2   (list left right))
3 (defun left-branch (mobile)
4   (first mobile))
5 (defun right-branch (mobile)
6   (second mobile))
7 (defun make-branch (len structure)
8   (list len structure))
9 (defun branch-len (branch)
10   (first branch))
11 (defun branch-structure (branch)
12   (second branch))
13 
14 (defun structure-is-weight? (structure)
15   (atom structure))
16 (defun weight-of-branch (branch)
17   (let ((struct (branch-structure branch)))
18     (if (structure-is-weight? struct)
19 	struct
20 	(weight-of-mobile struct))))
21 (defun weight-of-mobile (mobile)
22   (+ (weight-of-branch (left-branch mobile))
23      (weight-of-branch (right-branch mobile))))
24 (defun torque-of-branch (branch)
25   (* (branch-len branch)
26      (weight-of-branch branch)))
27 (defun branch-balanced? (branch)
28   "A branch is balanced either when it has a structure
29   that's a simple weight, or when the structure is
30   a balanced mobile" 
31   (let ((struct (branch-structure branch)))
32     (or
33      (structure-is-weight? struct)
34      (mobile-balanced? struct))))
35 (defun mobile-balanced? (mobile)
36   (let ((lb (left-branch mobile))
37 	(rb (right-branch mobile)))
38     (and
39      (= (torque-of-branch lb)
40 	(torque-of-branch rb))
41      (branch-balanced? lb)
42      (branch-balanced? rb))))
43