The tree is displayed as an indented list. The line properties of the tree<-link
are used to determine the thickness, texture and colour of the lines.
Each node will, if it has sons, draw a line from (x + tree<-level_gap/2,
bottom side) down to halfway the last son. If to node is not the root,
it will draw a line from halfway its height towards the line from its
parent. See also node<->collapsed.
Note: the default tree is displayed left-to-right rather than
top-to-bottom as the nodes are often much wider than high. Using
left-to-right layout allows for displaying much larger hierarchies.
When a large number of items is to be displayed as an indented list,
also consider class list_browser.
The indentation may be realised using
list_browser->tab_stops.
