I have Implemented a PanelType structure that includes an idname, a label, a parent, and a children list. The parent and children relation is used to create a hierarchy of PanelTypes. The functions WM_paneltype_add and WM_paneltype_remove allow adding and removing PanelTypes to/from the hierarchy. When a PanelType is added, it is also inserted in the children list of its parent. When a PanelType is removed, it is also removed from the children list of its parent. The parent and children relationship can be used to organize PanelTypes in a tree-like structure.