Method convert

base/src/main/java/net/time4j/Duration.java:3430–3441

(
        TimeSpan<U> timespan
    )

    }

    private static <U extends IsoUnit> Duration<U> convert(
        TimeSpan<U> timespan
    ) {

        if (timespan instanceof Duration) {
            return cast(timespan);
        } else {
            Duration<U> zero = ofZero();
            return zero.plus(timespan);
        }

    }

    private boolean isFractionUnit(IsoUnit unit) {