ClipContext *cc = clip->getContext();

      radius = thing->radius; // where force goes to zero
      cc->bbox[BOXTOP]    = thing->y + radius;
      cc->bbox[BOXBOTTOM] = thing->y - radius;
      cc->bbox[BOXRIGHT]  = thing->x + radius;
      cc->bbox[BOXLEFT]   = this->x - radius;

      xl = (cc->bbox[BOXLEFT]   - bmaporgx - MAXRADIUS) >> MAPBLOCKSHIFT;
      xh = (cc->bbox[BOXRIGHT]  - bmaporgx + MAXRADIUS) >> MAPBLOCKSHIFT;
      yl = (cc->bbox[BOXBOTTOM] - bmaporgy - MAXRADIUS) >> MAPBLOCKSHIFT;
      yh = (cc->bbox[BOXTOP]    - bmaporgy + MAXRADIUS) >> MAPBLOCKSHIFT;

      for (bx = xl; bx <= xh; bx++)
      {
         for(by = yl; by <= yh; by++)
            P_BlockThingsIterator(bx, by, PIT_SOMEFUNCTION, cc);
      }

      cc->done();