The last of the four routines of MOVE_TABLE, for an object facing -U (bits 6-7 of +6 = 3). The object's footprint is its U and V plus or minus its half-sizes (+8, +9); its leading edge is at U less the half-size in U. The cells to test are the ones that edge's two corners stand in now, and, if this turn's step in U (+A, negative) takes the edge into the next column, the ones its corners reach there -- at most four, each tested once. For each, CUT_STEP_MINUS_U checks the object's box, moved by the step, against the cell's boxes and cuts the step short at the first it runs into; the first cut ends the search, with carry set.

The corners only choose which cells to look in: the box test itself works from the object's own position and step (PLACE_IN_CELL), so a cell tested twice or one the object is nowhere near costs time but cannot give a wrong answer.