ALL: Set this keyword if you wish all the names to be selected
initially.
CANCEL: An output keyword set to 1 if the user cancels or quits the
program without hitting the Accept button. Set to 0 if a proper
selection was made and the use hits the Accept button.
COUNT: An output keyword containing the number of elements in the return array.
GROUP_LEADER: The widget identifier of a widget who will be the group leader
for this dialog. Passing a group leader is the *only* way to
assure the dialog will be a MODAL dialog (as opposed to a blocking
dialog). A GROUP_LEADER is required if you will be using this
function in an IDL Virtual Machine application.
LABEL: A string that will be placed on a label above the selections.
If not used, no label is used in the program.
NUMCOLS: The number of columns to organize the string array in. The default
is to use one column per approximately 20 strings.
TITLE: A string that is used for the title of the dialog window. If
undefined, then "Selection Widget" is used.