class dtd_notation :string -> Pxp_core_types.I.ext_id -> Pxp_core_types.I.rep_encoding ->object..end
 new dtd_notation a_name an_external_ID init_encoding method name : string
method ext_id : Pxp_core_types.I.ext_id
method encoding : Pxp_core_types.I.rep_encoding
        