def currency
      # Mixing currency not allowed; this
      # library can't convert between
      # currencies.
      currencies = @contents.map { |item| item[:currency] }.uniq || "USD"

      case currencies.count
        when 0
          "USD"
        when 1
          currencies.first
        else
          raise RuntimeError.new("Mixing currency not allowed")
      end

    end
def currency
      # Mixing currency not allowed; this
      # library can't convert between
      # currencies.
      currencies = @contents.map { |item| item[:currency] }.uniq || "USD"

      case currencies.count
        when 0
          "USD"
        when 1
          currencies.first
        else
          raise RuntimeError.new("Mixing currency not allowed")
      end

    end
Returns the currency for the cart.  Mixing currency not allowed; this
 library can't convert between currencies.