(legal example)
(Main program defined with numeric O-word title, terminated by EOF)
O2 sub (subprogram begin) (program end signaled by O2 sub)
(in sub 2)
G0 Z3
(returning from sub 2)
O2 endsub

O1 (main program begin)
G0 X1Y2
(calling sub 2)
O2 call
(returned from sub 2)

(end of main signaled by EOF)