7 Hierarchical List Control
| (require mrlib/hierlist) | package: gui-lib | 
A hierarchical-list% control is a list of items, some of which can themselves be hierarchical lists. Each such sub-list has an arrow that the user can click to hide or show the sub-list’s items.
The list control supports the following default keystrokes:
- Down: move to the next entry at the current level (skipping lower levels). 
- Up: move to the previous entry at the current level (skipping lower levels). 
- Left: move to the enclosing level (only valid at embedded levels). 
- Right: move down in one level (only valid for lists). 
- Return: open/close the current selected level (only valid for lists).