.. efun:: int set_heart_beat(int flag)

  Enable or disable heart beat. The driver will apply the lfun :applied:`heart_beat` to the current object every :macro:`__HEARTBEAT_INTERVAL__` seconds, if it is enabled. A shadow over the :applied:`heart_beat` lfun will be ignored.

  If the heart beat is not needed for the moment, then do disable it. This will reduce system overhead.

  Return true for success, and false for failure.

  Disabling an already disabled heart beat (and vice versa enabling and enabled heart beat) counts as failure.

  Note that :applied:`heart_beat` is called only if there are interactive players in the game.

  .. todo:: confirm above, I think there was a related change

  .. seealso:: :applied:`heart_beat`, :efun:`call_out`