+type uri = string
+type position = string
+type depth = int option
+type sort = string
+
+type r_obj = (uri * position * depth)
+type r_rel = (position * depth)
+type r_sort = (position * depth * sort)
+
+type must_restrictions = (r_obj list * r_rel list * r_sort list)
+type only_restrictions =
+ (r_obj list option * r_rel list option * r_sort list option)
+
+
+