public GroupingOperation addChild(GroupingOperation op) {
        op.parent = this;
        children.add(op);
        return this;
    }
Adds a child grouping node to this. This will also set the parent of the child so that it points to this node.

@param op the child node to add
@return this, to allow chaining
container-search/src/main/java/com/yahoo/search/grouping/request/GroupingOperation.java