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appinfo considered harmful. Re: What is the advantage of RELAX incomparison to Schemas?

David E. Cleary wrote:

> > I'd like to think that the Schema WG might reconsider its decision to
> > omit context-aware constraints, though I'm aware that it's unlikely to
> > happen any time soon.
> In the meantime, XML Schema has well defined extension mechanisms through
> either the appInfo element or the use of non-native attributes. These can
> added directly to an XML Schema with affecting XML Schema validity. An
> application or second level validator can then use this information to
> validate context-aware constraints. Instead of waiting for the XML Schema
> to create this functionality for a future version of XML Schema, create
> yourself today so that the Schema WG can use it as input for the next
> version.
> This should be done for any feature anyone believes should be part of XML
> Schema but isn't.

The great danger of this is that all sorts of proprietary appinfo based
constructs are sure to proliferate which have only a local meaning and will
render such documents uninterpretable by common software. For example, there
would be no consensus as to the post schema-validation infoset for a
particular document. This is bad for XML (IHMO). Perhaps my reading is
incorrect but I don't specifically see a prohibition on changes to the PSVI
from appinfo compontents.