public void associateWithGroupNode(GeometryGroupNode node, int startIndex) {
        if (isGrouped()) {
            unassociateFromGroupNode();
        }

        this.groupNode = node;
        this.startIndex = startIndex;
    }
Associate this <code>Geometry</code> with a {@link GeometryGroupNode}.

Should only be called by the parent {@link GeometryGroupNode}.

@param node Which {@link GeometryGroupNode} to associate with.
@param startIndex The starting index of this geometry in the group.
jme3-core/src/main/java/com/jme3/scene/Geometry.java