Summ of combinations

\* <---MATH STUFF ---> *\

(define fac
  0 -> 1
  N -> (* N (fac (- N 1))))

(define combination
  N K -> (/ (fac N) (* (fac (- N K))
               (fac K))))

(define sum-to-n
  N 0 -> (combination N 0)
  N K -> (+ (combination N K)
        (sum-to-n N (- K 1))))

(define c-sum
  N -> (sum-to-n N N))

\* </---MATH STUFF ---> *\