datatype coins = Penny | Nickel | Dime | Quarter

fun compareCoins (Quarter : coins, Dime : coins) = 2.5
  | compareCoins (Dime, Nickel) = 2.0
  | compareCoins (Nickel, penny) = 5.0
  | compareCoins (Quarter, Nickel) =
       compareCoins (Quarter, Dime) * compareCoins (Dime, Nickel)
  | compareCoin (Quarter, Penny) =
       compareCoins (Quarter, Nickel) * compareCoins (Nickel, Penny)
  | compareCoin (Dime, Penny) =
       compareCoins (Dime, Nickel) * compareCoins (nickel, Penny)
  | compareCoins (x, x) = 1.0
  | compareCoins (x1, x2) = 1 / compareCoins (x2, x1)