initialization.
* Triggered only once before the screen is presented to a user for the first time
* can be used to initialize selection-screen comments, pushbuttons with values.

at selection-screen.
* Triggered every time a menu option selected, application toolbar pressed, 
* selection-screen pushbutton pressed, <enter> key pressed.

at selection-screen output.
* (1) Triggered after initialization;
* (2) Triggered after at selection-screen event or any of its variant   
*     like at selection-screen on <field> etc. to present refreshed the screen to a user.
*     It can be used to change the properties of selection-screen elements. 

at selection-screen on <field>.
at selection-screen on block <block name>. 
at selection-screen on radiobutton group <radiobutton group name>.
at selection-screen on value-request for <field>. 
at selection-screen on help-request for <field>.

" position 
selection-screen comment 1(20) lb01 
   for field t01 visible length 21 
   modif id g01 for table node id id1.

" position low/high
selection-screen comment pos_low (20) text-012 
   for field t02 visible length 21 
   modif id g02 for node node id id2.

selection-screen comment pos_high (20) text-013 
   for field t03 visible length 23 
   modif id g01 for node node id id3.