Describe negated shortcut content types

[ndw]

Fix #1115

Note how I've proposed to handle negation of exclusions.

[gimsieke]

makes sense

[xml-project]

I am not sure, I got your example right.
The spec says:

The document is considered acceptable if and only if it matches at least one acceptable content type and the last content type that matched was not forbidden.

Now consider a port defined with "-xml". A document marked with "text/plain" should be accepted, but: It does not match any acceptable content type.
What do I miss?

Additionally I think we need to say something about "-any": Is it any error or is it simply ignored? Or is it a new feature to make sure, NO document is supplied on a sequence port?

[gimsieke]

Maybe we need to say that if the list only expands to exclusions, */* will be prepended?

[ndw]

It's an interesting question. I think that just "doesn't match at least one acceptable type". Just saying -application/xml wouldn't match text/plain either. If you don't have any positive matches, I think that's just not going to work. When we actually use it in the Invisible XML step, we write:

any -xml -html

[ndw]

There are already ways to make a port that doesn't accept any documents, I don't think we have to call attention to -any, but we could.

[xml-project]

@ndw Ah, ok! I obviously missed the main point. Now your proposal makes totally sense to me. Lets see how often forget about this point when writing pipelines.