(legal example)
(Main program defined with numeric O-word title, terminated by M30)
O1 (main program begin)
G0 X1Y2
(calling sub 2)
O2 call
(returned from sub 2)
M30 (end of main)
(should never get here)

O2 sub (subprogram begin) (program end signaled by O2 sub)
(in sub 2)
G0 Z3
(returning from sub 2)
O2 endsub
