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                     Add part to part list (left mouse button 
 and CTRL key) Pressing the CTRL key on the keyboard and the left mouse buttonat the same time
 denotes the following:
                     
                    
                        - If the part does 
 not belong to the part list, it is added. If the list was previously 
 empty, the active part is added to the list as well!
 
                        - If the part is already
 part of the part list, it is removed from the list. If only the active 
 part is in the list after this removal, it is also removed from the list!
 
                        - If a superordinate 
 part of the clicked part is already in the list, the click is ignored.
 Reason: All sub-parts are in the list together with their superordinate 
 part, which means that the subordinate part cannot be removed from the 
 list. 
                     
                    If you left-click an empty area of the drawing area or the ICN, all
 part lists are deleted.  
                    
                          If there is one single, superordinate part in the part structure (e.g. a main assembly), which is included in a multiple selection (e.g. after CTRL+LMB and drawing of a selection rectangle), the selection in the drawing area will be handled as if only one single part had been selected. If a multiple selection context menu is required, all parts beneath the superordinate part of a multiple selection can be added, without selecting the single, superordinate part.  
                    ☑ ICN ☑ Drawing
                     
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