How can I use the /multiple keyword with WIDGET_LIST

[Nicholas Keat <nick(at)impactscan.org>]

I have created a widget based application for processing CT scanner
image files that allows the user to select images from a list, and
analyse them for certain image quality parameters.

What I would like to be able to do is to select, say, three images from
a list of ten for analysis.  From the IDL documentation (ha!) it seems
that I can use the '/multiple' keyword with widget_list to allow
multiple selections, which it seems to do.  Running my app with a
/multiple added to my widget_list statement allows me to press 'Shift'
and 'Ctrl' to highlight more than one element on the list.  The problem
is that the events generated by making multiple selections seem to be
exactly the same as those created from making single selections.  How do
I tell if the user has pressed 'Shift' or 'Ctrl' to make an extended
selection?  Is it possible without extended hacks to track key presses?
I've looked at the usual source of IDL explanations (www.dfanning.com)
without finding any references...  Surely someone else must have had to
do this before!

Nick
-- 
Nicholas Keat
Senior Physicist - ImPACT Group, St George's Hospital
London SW17 0QT
Tel +44 (0)20 8725 3366       http://www.impactscan.org