-- The factorial function:
fac :: Int -> Int
fac n = if n==0 then 1 else n * fac (n - 1)

-- The last element of a list:
last xs | _ ++ [x] == xs  = x    where x free

-- The last element of a list with a functional pattern:
last' (_ ++ [x]) = x

perm []                = []
perm (xs ++ [x] ++ ys) = x : perm (xs ++ ys)