public Grouping addLevel(GroupingLevel level) {
        groupingLevels.add(Objects.requireNonNull(level));
        return this;
    }
Appends the given grouping level specification to the list of levels.

@param level the level to add
@return this, to allow chaining
@throws NullPointerException if <code>level</code> argument is null
searchlib/src/main/java/com/yahoo/searchlib/aggregation/Grouping.java