Position previousCandidate(const Position& position)
{
    PositionIterator p = position;
    while (!p.atStart()) {
        p.decrement();
        if (p.isCandidate())
            return p;
    }
    return Position();
}
Position nextCandidate(const Position& position)
{
    PositionIterator p = position;
    while (!p.atEnd()) {
        p.increment();
        if (p.isCandidate())
            return p;
    }
    return Position();
}