public SortedSet<Integer> getTagPositions() {
        final SortedSet<Integer> tagPositions = new TreeSet<>();

        populateTagPositionCollection(tagPositions);

        return tagPositions;
    }
@return The bit mask as a list of field positions where the field is
rolled up, e.g. [0,2] indicates that the fields at position 0 and
2 are rolled up
stroom-statistics/stroom-statistics-impl-sql/src/main/java/stroom/statistics/impl/sql/rollup/RollUpBitMask.java