1st way:
--------

pre-handler:
   if fall-through key is pressed or normal direction says player is underneath
      return IGNORE
   else
      return ACCEPT_ONCE // so pre-handler is called again next-step incase key is pressed.


2nd way:
--------

pre-handler:
   if normal direction says player is underneath, or fall-through key is pressed
      return IGNORE
   else
      return ACCEPT

ongoing-handler:
   if fall-through key is pressed
      foreach arbiter in callback object, arbiter.state = IGNORE