+/* rsbetween is (mostly) used for xml searches. It returns the hits of the
+ * "middle" rset, that are in between the "left" and "right" rsets. For
+ * example "Shakespeare" in between "<title>" and </title>. The thing is
+ * complicated by the inclusion of attributes (from their own rset). If attrs
+ * specified, they must match the "left" rset (start tag). "Hamlet" between
+ * "<title lang=eng>" and "</title>". (This assumes that the attributes are
+ * indexed to the same seqno as the tags).
+*/