Method update

base/src/main/java/net/time4j/Duration.java:4282–4321

(
            long amount,
            long factor
        )

        }

        private void update(
            long amount,
            long factor
        ) {

            if (amount >= 0) {
                IsoUnit unit = NANOS;

                for (int i = this.items.size() - 1; i >= 0; i--) {
                    Item<IsoUnit> item = this.items.get(i);
                    if (item.getUnit().equals(NANOS)) {
                        this.items.set(
                            i,
                            Item.of(
                                MathUtils.safeAdd(
                                    MathUtils.safeMultiply(amount, factor),
                                    item.getAmount()
                                ),
                                unit
                            )
                        );
                        return;
                    }
                }

                if (amount != 0) {
                    this.items.add(
                        Item.of(
                            MathUtils.safeMultiply(amount, factor),
                            unit
                        )
                    );
                }

            } else {
                throw new IllegalArgumentException(
                    "Illegal negative amount: " + amount);
            }

        }

        private void millisCalled() {