The system variable that controls the cumulative selection is PICKADD, the initial value for PICKADD is 2, which means the PICKADD is on; when the value is 0 or 1, the PICKADD is off, which means the previously selected objects are removed from the selection set.
With Symmetric Draw in GstarCAD, things got much easier. We can directly draw symmetric shapes. That is to say, when we draw the fist half, you got another half automatically drawn. No need to copy or mirror.