<schemaSpec>/@source needs a "tei:none" value and should be mandatory

[martindholmes]

Coming from the ATOP work with @HelenaSabel and @sydb:

The @source attribute on <schemaSpec> is used to specify which TEI base odd version you want your schema to be compiled against (or the URL of another base odd at a specified URL). When it is not present, we assume the value of tei:current, the last stable release of TEI.

However, as ODD is increasingly used for non-TEI schema descriptions (something we support and encourage), it is increasingly common for the base TEI ODD not to be needed or wanted; in such cases, it is retrieved and processed pointlessly, and might even result in unexpected interference in the processing of an ODD.

For this reason, we propose two things:

1. We should add and document a new value of @source="tei:none", meaning that no base TEI ODD is to be retrieved at all. This simplifies and streamlines processing in cases where no base ODD is required.
2. We should make @source on <schemaSpec> mandatory, so that no-one omits it by accident; it is all too easy to be taken by surprise when a new release of TEI suddenly results in changes to your schema that you did not expect, just because you omitted @source and so "tei:current" was assumed.

[peterstadler]

I agree with 1. and 2. but would recommend a different value for @source to declare that this ODD is not based on anything else. Since this is not a TEI feature, I'd try to avoid the "tei" prefix and simply label it as "no-source" or "odd:origin" …