private SegmentStartPoint findLastStartPoint() {
        for (int i=segmentStartPoints.size()-1; i>=0; i--) {
            SegmentStartPoint startPoint=segmentStartPoints.get(i);
            if (startPoint!=null)
                return startPoint;
        }
        return null; // Impossible
    }
Returns the next open segment to explore, or null if no more segments exists or should be explored
searchlib/src/main/java/com/yahoo/searchlib/ranking/features/fieldmatch/FieldMatchMetricsComputer.java