-- | The standard factorial function. See
-- <http://www.willamette.edu/~fruehr/haskell/evolution.html> for
-- possible improvements.
+--
+-- Examples:
+--
+-- >>> factorial 0
+-- 1
+--
+-- >>> factorial 4
+-- 24
+--
factorial :: Int -> Int
factorial n
| n <= 1 = 1