Used by the routines at LEAPER, REMOTE_ROBOT, SLOW_CHASER, CLOCKWORK_MOUSE, CEILING_DROP, LOOSE_VALVE, WANDERER, PACER, SPARK_CHASER, SHUTTLE_U, DROPPING_BLOCK, PUSHABLE, LIFT and EXTRA_LIFE.

Takes one off the step in Z -- the pull of gravity -- then lets ADJ_FOR_OUT_OF_BOUNDS cut the steps down to what fits, and adds what is left to the position. The player's move (MOVE_PLAYER) comes in at ADD_DUVZ, having cut the move itself. As Pentagram's DEC_DZ_AND_UPDATE_UVZ; Knight Lore's add_dXYZ.