presto-main/src/main/java/io/prestosql/type/DecimalOperators.java/DecimalOperators/internalSubtractLongLongLong
class DecimalOperators:    private static Slice internalSubtractLongLongLong(Slice a, Slice b, int rescale, boolean rescaleLeft)
    {
        try {
            Slice tmp = unscaledDecimal();
            if (rescaleLeft) {
                rescale(a, rescale, tmp);
                UnscaledDecimal128Arithmetic.subtract(tmp, b, tmp);
            }
            else {
                rescale(b, rescale, tmp);
                UnscaledDecimal128Arithmetic.subtract(a, tmp, tmp);
            }
            throwIfOverflows(tmp);
            return tmp;
        }
        catch (ArithmeticException e) {
            throw new PrestoException(NUMERIC_VALUE_OUT_OF_RANGE, "Decimal overflow", e);
        }
    }