Used by the routines at DO_LOOK, DO_DROP, DO_TAKE, MOVE, LOOK_THROUGH, GO_THROUGH, DO_TALK, DO_DIG, DO_SHOOT, DO_INVENTORY, DO_OPEN, DO_CLOSE, DO_ATTACK, DO_PUT_IN, DO_DRINK, DO_EAT, DO_STRIKE, DO_GIVE, DO_EXAMINE, DO_EMPTY, INTO_THE_RIVER, DO_LOCK, THROW_THROUGH, DRINK_WATER, DO_TIE, DO_UNTIE, SWIM_RIVER, DO_BURN, SWIM_BLACK_RIVER, DRINK_BLACK_WATER, WEAR_RING, TAKE_OFF_RING, DO_CAPTURE, GIVEN_SOMETHING, GANDALF_WHATS_THIS, GANDALF_CHATTER, THORIN_THRAINS_KEY, ELROND_HELLO, DO_CLIMB_OUT, DO_CLIMB_INTO, WARG_HOWLS, THORIN_WHERES_THIEF, THORIN_CHATTER, DRAGON_FOLLOWS, DRAGON_THREATENS, DRAGON_HUNTS, MAGIC_DOOR_EXAMINED, ELROND_READS_MAP, THROW_ROPE_ACROSS, PULL_ROPE, GOLLUM_ASKS, GOLLUM_HEARS_ANSWER, GOLLUM_POCKETS, TROLLS_EAT, TROLLS_TURN_TO_STONE, TROLLS_TALK, ELROND_GIVES_LUNCH and JUMP_ONTO_BARREL.

With DOING_IT set this returns and the handler goes on to do the action. With it clear it sets SUCCEEDED -- yes, it would work -- and drops its own return address, so the RET leaves the handler that called it.