Returns a tuple of (total amount deposited, total amount
        withdrawn).

def balance(self):
        """Returns a tuple of (total amount deposited, total amount
        withdrawn)."""
        sin = Decimal("0.00")
        sout = Decimal("0.00")

        for t in self.trans:
            if t.amount < Decimal("0.00"):
                sout += t.amount
            else:
                sin += t.amount

        return sin, sout