(define (reduce op start-value l)
  (cond ((null? l) start-value)
        (else (op (car l) (reduce op start-value (cdr l))))))

(define (all? pred? l)
  (reduce (lambda (x y) (and x y)) #t (map pred? l)))

(define (nth index list)
  (cond
    ((= index 0) (car list))
    (else (nth (- index 1) (cdr list)))))


(define (iter n list result)
    (cond
      ((null? list) result)
      ((= n 0) result)
      (else (iter (- n 1) (cdr list) (cons (car list) result)))))

(define (take n l)
  (reverse (iter n l (list))))

(define (underD mat)
  (map (lambda (n) (take n (nth n mat))) (range 1 (length mat) 1)))

(define (triangular? mat)
  (all? (lambda (row) (all? (lambda (x) (= x 0)) row)) (underD mat)))