null [] = True
null (_:_) = False
--- | 'length' returns the length of a finite list as an 'Int'.
+-- | /O(n)/. 'length' returns the length of a finite list as an 'Int'.
-- It is an instance of the more general 'Data.List.genericLength',
-- the result type of which may be any kind of number.
length :: [a] -> Int