The -selectmode option of the BWidget ListBox is a readonly option.
Meaning that you can specify its value when the listbox is created, but
after it's been created, all attempts to change it will simply do
BWidgets differs from Tk in this respect. Tk will respond with an
error if an option you're trying to configure cannot be configured
after widget creation. BWidget just silently ignores you.
(bin) 49 % package require BWidget
(bin) 50 % ListBox .l -selectmode single
(bin) 51 % .l cget -selectmode
Hope that helps!