La cochina más grande que he programado en mi vida

(define (sup-izq mat)
(cond ( (null? mat)
'())
( (even? (length mat))
(spx mat))
(else
(spx (conv mat)))))
(define (conv mat)
(map reverse (map (lambda(x) (cdr x))
(map reverse
(reverse (cdr (reverse mat)))))))
(define (spx mat)
(obtener (/ (length mat) 2)
(map (lambda(x) (obtener (/ (length mat) 2) x ))
mat)))
(define (obtener ele vec)
(cond ((or (zero? ele)
(null? vec))
'())
(else
(cons (car vec) (obtener (- ele 1) (cdr vec))))))