Used by the routines at SET_WIPE_AND_DRAW_FLAGS and TOP_FOLLOWS_LEGS.

Called once an object has moved or changed its drawing: by the robot's top (TOP_FOLLOWS_LEGS), and through SET_WIPE_AND_DRAW_FLAGS -- which sets bits 4 and 5 of the object's flags first -- by the legs and most other update routines. It works out the object's new screen rectangle (CALC_2D_INFO), forms the smallest rectangle covering that and the one it had at the start of the turn (+$1C to +$1F, copied there by the main loop, AFTER_GAME), and walks all 56 records setting the draw flag, bit 4 of +$07, on every live object whose rectangle meets it -- the moving object itself included.

Across, the rectangles are measured in byte columns (pixel x over 8); up, in pixel rows from the bottom of the screen. E is the union's first column and D its width in columns; L its lowest row and H its height. Only the moving object's rectangle is used: an object redrawn because it meets it does not in turn mark the objects that meet it. Pentagram's routine, instruction for instruction.