Resposta ao exercício 13

(defun igual? (x y)
  (igual-aux x x y y))

(defun igual-aux (x+ x- y+ y-)
  (cond ((zerop x+) (zerop y+))
        ((zerop x-) (zerop y-))
        ((or (zerop y+) (zerop y-)) nil)
        (t (igual-aux (1+ x+) (1- x-) (1+ y+) (1- y-)))))