The XDo interface makes uses of the Motif "List Widget" to allow the user
to select one or several items among a list of text choices. 
Two selection policies may be used, namely Single Selection and Extended
Selection:

--Single Selection may be used when only one of many choices may be 
  selected at a time. For instance, the Pushbutton "Open" of the menu "File"
  of the MenuBar pops up a window containing a list of MIDAS Tables from  
  which only one may be selected and opened. 
  The selection is done by dragging  and releasing the mouse button over the
  selected item.



--Extended Selection allows any number of items to be selected in 
  noncontiguous ranges. For instance, the Pushbutton "Select Cols" pops up
  a window containing the list of columns of the OST. Any of these columns
  may be selected and displayed in the scrolled windows.
  Contiguous items may be selected by dragging and releasing the mouse button.
  Pressing the Ctrl-C key  before selecting new items avoid to deselect any
  other selected item. 
