docs: def binom(n, k):
      """
      Returns binomial coefficient (n choose k).
      """
      # http://blog.plover.com/math/choose.html
      if k > n:
          return 0
      if k == 0:
          return 1
      result = 1
      for denom in range(1, k + 1):
          result *= n
          result /= denom
          n -= 1
      return result