fallingFactorial :: Integral a => a -> a -> a
fallingFactorial m n = product [m - n + 1 .. m]

--
-- 階乗
--
factorial :: Integral a => a -> a
factorial m = product [2 .. m]

--
-- 整数の桁数を求める
--
digits ::  (Integral a, Show a) => a -> Int
digits  = lengt
permutationSize :: Integral a => a -> a -> a
permutationSize = fallingFactorial
permutationSize = fallingFactorial