-- test whether a nat. number is a prime
prime :: Integer -> Bool
prime n = n >= 2 
  && not (divisible_range n 2 (n - 1))

-- divisible_range x n m tests
--   whether x divisible by any k within 
--   n <= k <= m
divisible_range :: Integer -> Integer 
  -> Integer -> Bool
divisible_range x n m = 
  if n > m then False
  else divisible_range x (n+1) m
      || is_divisible x n
      
is_divisible x n = x `mod` n  == 0