Graphics 136 to 139: a block that crumbles a stage for each turn he stands on it, and vanishes after the fourth. It needs both bit 7 of +17, which the collision code sets when he (or the heavy block, HEAVY_BLOCK) lands on it (MARK_STOOD_ON), and bit 3 of +D, set for anything landing on it (ADJ_DZ_FOR_OBJ_INTERSECT); it clears both, then moves up a graphic, or from 139 becomes graphic 1, which DRAW_OBJECT rubs out and turns into an empty record.

Measured in the simulator in room 4: he landed on it and it went 136, 137, 138, 139, gone, a turn each, and he fell.