FW: [xml-dev] having problems with the "|" in field's element in the xs:key el ement

John's response was to a question I posted to him offline.  I'll repost it
here.  I hope someone can give me an answer that doesn't require severe
remedial total-immersion math courses to understand.

> I'm trying to understand why this is so:
> 
> If a document validator can know if a field occurrence is 
> optional in an instance's corresponding content model, then 
> how is it that a schema validator cannot know that a declared 
> key field is optional in its corresponding content model?  
> 
> The only thing I can think of that's different is that the 
> document validator may be able to narrow down the applicable 
> content models based on components that've already be read 
> from the document.