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).