public boolean isFullyCoveredBy(ColumnFamily cf, long now)
    {
        // cf will cover all the requested columns if the range it covers include
        // all said columns
        CellName first = cf.iterator(ColumnSlice.ALL_COLUMNS_ARRAY).next().name();
        CellName last = cf.reverseIterator(ColumnSlice.ALL_COLUMNS_ARRAY).next().name();
        return cf.getComparator().compare(first, columns.first()) <= 0
            && cf.getComparator().compare(columns.last(), last) <= 0;
    }