
An "unused variable" is a formal parameter that is declared
by a function but never used in the body.

List the unused variables in each of these little language
expressions:


(f x)                      (lambda (x)
                              y)


(lambda (x)                ((lambda (x) x)
  (f x))                    (lambda (x) y))


                           (a
                            (lambda (z)
(lambda (x)                   (lambda (y)
  (lambda (x)                   (lambda (x)
    x))                           x))))
