; returns n by n matrix with d on diagonal and b elsewhere.
(lambda (n d b) (let ((y (let r ((k n))
     (if (= k 0) '() (cons b (r (- k 1)))))))
   (let M ((m 0)(y y)) (if (= m n) '() (cons
     (let r ((k m)) (if (= k 0)
       (cons d (cdr y)) (cons b (r (- k 1)))))
     (M (+ m 1)(cdr y)))))))