Add new predicate for required column for entry Namespaces in StructTreeRoot

[MaximPlusov]

ISO 32000-2:2020, Table 354: Namespaces - Required if any structure elements have namespace identifiers.

Maybe it is possible to add new predicate to check this requirment?

There may be many such cases not represented in the Arlington. Maybe they need to be documented somehow?

[petervwyatt]

I think we'd have to invert that requirement by adding a "SpecialCase" predicate to NS in StructureElement that asserts something like fn:IsPresent(trailer::Catalog::StructTreeRoot::Namespaces).

AFAICT there also isn't any strict "shall" requirement in ISO 32000-2 which says the NS in StructureElement has to be an element in the StructTreeRoot Namespaces array! At most, there is a descriptive semi-factual statement in Table 354: "An array of namespaces used within the document.". This possibly seems like an oversight...

[petervwyatt]

See also PDF Errata #396