public Group addChild(Group child) {
        if (child == null) {
            throw new IllegalArgumentException("Child can not be null.");
        }
        children = add(children, child);
        return this;
    }
Adds a child group to this.

@param child The group to add.
@return This, to allow chaining.
searchlib/src/main/java/com/yahoo/searchlib/aggregation/Group.java