STOP

Ends the running of the procedure in which it appears. Control is returned to the context in which that procedure was invoked. The stopped procedure does not output a value.

TO MYPROGRAM :arg
  PRINT [Before Stop]
  IF 1=:arg [STOP]
  PRINT [After Stop]
END

MYPROGRAM 1
Before Stop

MYPROGRAM 2
Before Stop
After Stop