+(* output data structures ***************************************************)
+
+type path = string * (string list) (* the name of an attribute *)
+
+type value = string list (* the value of an attribute *)
+
+type attribute = path * value (* an attribute *)
+
+type attribute_group = attribute list (* a group of attributes *)
+
+type attribute_set = attribute_group list (* the attributes of an URI *)
+
+type resource = string * attribute_set (* an attributed URI *)
+
+type resource_set = resource list (* the query result *)
+
+type result = resource_set
+
+