MoneyKentBeck.hs

data MoneyExpr                  -- An Money expression made of EITHER:
  = KnownAmount Money           -- * A known amount and currency (base case)
  | MoneyAdd [MoneyExpr]        -- * A sum of several money expression
  | MoneyMul MoneyExpr Double   -- * A money expression multiplied by a factor
  deriving (Show, Eq)