N and M are not optional and specify the width and depth of the
area in average characters.
tree is a character array of nitems.
sel is the index of the item selected by the user.
icon_str is a character string giving a list of icon resources in the
options can take the keyword fixed_font which causes the control to use a
fixed font for the names of the tree nodes. This can be useful in cases where
the node names are intended to line up. options can also take the
keyword keep_focus to prevent the focus from shifting from the selected item.
This is similar to the effect produced by a list box.
If variable_size is included as one of the options, nitems is
treated as a variable that subsequently can be modified and updated by a call
to window_update@. However, the size cannot be increased above its initial
value. Since blank elements in the input array are ignored, the only reason to
use this feature is to reduce the processing time (and therefore possible
flicker) when using a very large array, most of whose elements are blank.
For further details see Tree-view.