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 9branch)
17 (let