public Group setId(ResultNode id) {
        this.id = id;
        return this;
    }
Sets the label to use for this group. This is a {@link ResultNode} so that a group can be labeled with
whatever value the classifier expression returns.

@param id the label to set
@return this, to allow chaining
searchlib/src/main/java/com/yahoo/searchlib/aggregation/Group.java