Method with

base/src/main/java/net/time4j/Duration.java:1537–1567

(
        long amount,
        U unit
    )

     * @see     #plus(long, IsoUnit) plus(long, U)
     */
    public Duration<U> with(
        long amount,
        U unit
    ) {

        long absAmount =
            ((amount < 0) ? MathUtils.safeNegate(amount) : amount);
        Item<U> item = replaceFraction(absAmount, unit);

        if (item != null) {
            absAmount = item.getAmount();
            unit = item.getUnit();
        }

        long oldAmount = this.getPartialAmount(unit);

        return this.plus(
            MathUtils.safeSubtract(
                MathUtils.safeMultiply(
                    absAmount,
                    (amount < 0) ? -1 : 1
                ),
                MathUtils.safeMultiply(
                    oldAmount,
                    this.isNegative() ? -1 : 1
                )
            ),
            unit
        );

    }

    /**
     * <p>Gets the absolute always non-negative copy of this duration. </p>