+ * subregions (if any) to replace the subrange here;
+ * NB have to resort subranges after doing this since e.g.
+ * [10-30 [12-20 [16-18], 13-19]]
+ * after deleting 12-20, 16-18 is promoted to sibling of 13-19
+ * but should follow it in the list of subranges of 10-30