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The PEN statements enable, disable, or suspend lightpen-event trapping.
 
PEN ON
PEN OFF
PEN STOP
 
Usage Notes
    ■ PEN ON enables lightpen-event trapping. A lightpen event occurs
      whenever the lightpen is activated by pressing the tip to the screen
      or pressing the touch ring. If a lightpen event occurs after a PEN ON
      statement, the routine specified in the ON PEN statement is
      executed.
    ■ PEN OFF disables lightpen-event trapping so no trapping takes place
      until another PEN ON statement is executed. Events occurring while
      trapping is off are ignored.
    ■ PEN STOP suspends lightpen-event trapping so no trapping takes place
      until a PEN ON statement is executed. Events occurring while trapping
      is suspended are remembered and processed when the next PEN ON
      statement is executed. However, remembered events are lost of PEN OFF
      is executed.
    ■ When a lightpen event trap occurs (that is, the GOSUB is performed),
      an automatic PEN STOP is executed so that recursive traps cannot take
      place. The RETURN from the trapping routine automatically executes
      a PEN ON statement unless an explicit PEN OFF was performed inside
      the routine.
    ■ If your program contains event-handling statements and you are
      compiling from the BC command line, use the BC /W or /V option.
      (The /W option checks for events at every label or line number; the
      /V option checks at every statement.) If you do not use these
      options and your program contains event traps, BASIC generates the
      error message, "ON event without /V or /W on command line."
    ■ PEN ON must be executed before you use the PEN function. If you try
      to execute the PEN function when the lightpen is off, BASIC generates
      the error message, "Illegal function call."
    ■ For more information, see Chapter 9, "Event Handling" in the
      Programmer's Guide.