Question about domains attribute

[Robert D Anderson <robander@us.ibm.com>]

The DITA spec is pretty clear that structural specializations need to declare @domains on the root element (specialization of map or topic), and that this attribute must describe the domains used in a given document type shell. The spec says of this attribute: "...the @domains attribute, whose value is a sequence of parenthesized module ancestry specifications. "
http://docs.oasis-open.org/dita/v1.2/os/spec/archSpec/domainsatt.html

The question is - what about if I have a document type shell with no domains? I've got a very simple map specialization - mostly a title, metadata, and very limited references to other maps. There is no need for any domains. I've declared the attribute for my root element, but in my DTD file the attribute is set to the empty string (""). This would not appear to comply with the language above, because the empty string is not a sequence of parenthesized modules. Is it correct to set this to the empty string? I've got a tool that reports an error here because @domains does not match the current definition. I can get around the error by setting a value like "none" or "(none)" but those clearly do not comply either.

So - what is the correct value for @domains in a document type shell that does not use any domains?

Robert D Anderson
IBM Authoring Tools Development
Chief Architect, DITA Open Toolkit (http://dita-ot.sourceforge.net/)