exception Internal_error of string
exception Cache_failure of string
exception Dtd_not_found of string (* dtd's url *)
+exception Key_already_in of string;;
+exception Key_not_found of string;;
+exception Http_client_error of string * string (* url, error message *)
+exception Unsupported_scheme of string (** unsupported url scheme *)
type encoding = [ `Normal | `Gzipped ]
type answer_format = [ `Text | `Xml ]