(rem num div)
remainder of dividing numerator by denominator.
user=> (rem 10 9)
1
user=> (rem 2 2)
0
;; rem and mod are commonly used to get the remainder.
;; mod means Gaussian mod, so the result is always
;; non-negative. Don't confuse it with ANSI C's %
;; operator, which despite being pronounced
;; 'mod' actually implements rem, i.e. -10 % 3 = -1.
user=> (mod -10 3)
2
user=> (rem -10 3)
-1