Listboxes changed quite a bit in Tk 4.0. See Chapter 32 for all the details. There are now four Motif-like selection styles, and two of these support disjoint selections. The tk_listboxSingleSelect procedure no longer exists. Instead, configure the selectMode attribute of the listbox. A listbox has an active element, which is drawn with an underline. It is referenced with the active index keyword.
The selection commands for listboxes have changed. Change:
$listbox select from index1
$listbox select to index2
$listbox select anchor index1
$listbox select set anchor index2
The set operation takes two indices, and anchor is a valid index, which typically corresponds to the start of a selection.
You can selectively clear the selection, and query if there is a selection in the listbox. The command to clear the selection has changed. It requires one or two indices. Change:
$listbox select clear
$listbox select clear 0 end