Schema instance documents

[Matthew Graham]

Hi,

Validation of the empty but correctly structured instance documents that
my XForms stylesheet produces is proving to be a bit of a problem with the
new schema. The problem is that the schema does not allow for empty values
for the attribute fields (dates, status, ivo-id), simple type elements
(e.g. xs:float) and enumerations.

Obviously when you are constructing the documents by hand, this is not a
problem but automated tools which process the schema have to know how to
handle such things. 

Some changes that would help are: 
- use of nillable, both for attributes and elements
- default values are specified for all enumerations

Of course, I could also amend my stylesheets to insert meaningful null
values where needed but I would like to have the discussion about whether
the schema should define nillable and default values extensively first.

	Cheers,

	Matthew