type Peg = String
type Move = (Peg, Peg)

hanoi :: Integer -> Peg -> Peg -> Peg -> [Move]
hanoi 1 a b c = [(a, c)]
hanoi n a b c = concat [
  hanoi (n-1) a c b,
  hanoi 1 a b c,
  hanoi (n-1) b a c ]