Seminario_TP4_EJ10

(defun cant (l)
    (if (null l)
        0
        (if (atom (car l))
            (+ 1 (cant (cdr l)))
            (cant (cdr l))
        )
    )
)

(defun reemplaza (l a b &optional (salida nil))
    (if (null l)
        salida
        (if (and (eq (car l) a) (> 1 (cant (cdr l))))
            (append (append salida (list b)) (cdr l))
            (reemplaza (cdr l) a b (append salida (list (car l))))
        )
    )
)

(defun ultimo (l &optional (salida nil))
    (if (null l)
        salida
        (if (atom (car l))
            (ultimo (cdr l) (car l))
            (ultimo (cdr l) salida)
        )
    )
)

(defun intercambia (l)
    (if (> 2 (cant l))
        l
        (if (atom (car l))
            (cons (ultimo l) (reemplaza (cdr l) (ultimo l) (car l)))
            (cons (car l) (intercambia (cdr l)))
        )
    )
)

(defun intercambiaTN (l)
    (if (atom l)
        l
        (mapcar 'intercambiaTN (intercambia l))
    )
)