public GroupingRequest setRootOperation(GroupingOperation root) {
        this.root = root;
        return this;
    }
Sets the root {@link GroupingOperation} that defines this request. As long as this remains unset, the request is
void.

@param root the root operation to set.
@return this, to allow chaining.
container-search/src/main/java/com/yahoo/search/grouping/GroupingRequest.java