In SAX.Writer xml: prefix not avaliable by default

[Maxim Reznik]

This code caused CONSTRAINT_ERROR with "Message: namespace is not mapped to any prefix"

      --  setting xml:lang attr
      Attrs.Set_Value
       (Namespace_URI => XML_URI,
        Local_Name    => Lang_Attribute,
        Value         => Self.Language);

--      Writer.Start_Prefix_Mapping
--        (Namespace_URI => XML_URI,
--         Prefix        => To_Universal_String ("xml"));

      Writer.Start_Element (Qualified_Name => Message_Element,
                            Attributes     => Attrs);

Vadim says Start_Prefix_Mapping should be optional.

[vadim.godunko]

Such error can appear only when writer is misused: Start_Document is not called before call to Start_Element.