287   Rows::iterator row_a = rows_.upper_bound(top);  in Subtract()  local
291   while (row_a != rows_.end() && row_b != region.rows_.end()) {  in Subtract()
293     if (row_a->second->bottom <= top) {  in Subtract()
296       MergeWithPrecedingRow(row_a);  in Subtract()
297       ++row_a;  in Subtract()
301     if (top > row_a->second->top) {  in Subtract()
305       assert(top <= row_a->second->bottom);  in Subtract()
307           row_a, Rows::value_type(top, new Row(row_a->second->top, top)));  in Subtract()
308       row_a->second->top = top;  in Subtract()
309       new_row->second->spans = row_a->second->spans;  in Subtract()
310     } else if (top < row_a->second->top) {  in Subtract()
313       top = row_a->second->top;  in Subtract()
322     if (row_b->second->bottom < row_a->second->bottom) {  in Subtract()
328           rows_.insert(row_a, Rows::value_type(bottom, new Row(top, bottom)));  in Subtract()
329       row_a->second->top = bottom;  in Subtract()
330       new_row->second->spans = row_a->second->spans;  in Subtract()
331       row_a = new_row;  in Subtract()
337     SubtractRows(row_a->second->spans, row_b->second->spans, &new_spans);  in Subtract()
338     new_spans.swap(row_a->second->spans);  in Subtract()
339     top = row_a->second->bottom;  in Subtract()
349     if (row_a->second->spans.empty()) {  in Subtract()
350       Rows::iterator row_to_delete = row_a;  in Subtract()
351       ++row_a;  in Subtract()
355       MergeWithPrecedingRow(row_a);  in Subtract()
356       ++row_a;  in Subtract()
360   if (row_a != rows_.end())  in Subtract()
361     MergeWithPrecedingRow(row_a);  in Subtract()