280   Rows::const_iterator row_b = region.rows_.begin();  in Subtract()  local
283   int top = row_b->second->top;  in Subtract()
291   while (row_a != rows_.end() && row_b != region.rows_.end()) {  in Subtract()
314       if (top >= row_b->second->bottom) {  in Subtract()
315         ++row_b;  in Subtract()
316         if (row_b != region.rows_.end())  in Subtract()
317           top = row_b->second->top;  in Subtract()
322     if (row_b->second->bottom < row_a->second->bottom) {  in Subtract()
326       int bottom = row_b->second->bottom;  in Subtract()
337     SubtractRows(row_a->second->spans, row_b->second->spans, &new_spans);  in Subtract()
341     if (top >= row_b->second->bottom) {  in Subtract()
342       ++row_b;  in Subtract()
343       if (row_b != region.rows_.end())  in Subtract()
344         top = row_b->second->top;  in Subtract()