Since a number of elements that were originally mandatory in MIX were, I 
believe, made optional at my request, I think I can answer this one.  
The MIX schema and the data dictionary do not align with respect to 
Obligation.  The reason for this is your (and our) situation.  The Data 
Dictionary expresses what *should* be present from a "best of all 
possible worlds", best practice, preservation standpoint.  However, the 
MIX schema recognizes that there are situations where users like 
ourselves may not have or be able generate all of the elements that the 
Data Dictionary specifies as mandatory and so it allows us still to 
express what we do have using MIX.  Simply as a practical matter, the 
schema is more lenient than the Data Dictionary with respect to obligation.

Rick Beaubien

Ed Fay wrote:
> There seems to be an inconsistency between the Data Dictionary and
> version 2 of the MIX schema. This may be intentional, in which case my
> apologies.
> In the Data Dictionary the element "Fixity" is defined as 'M'
> (mandatory), while in
> the element is defined: <xsd:element name="Fixity" minOccurs="0"
> maxOccurs="unbounded">. Which is correct?
> I am implementing some MIX where we only have pre-recorded fields to
> map, and no access to the files to generate checksums. I have the
> choice between omitting the "Fixity" element entirely, or putting it
> in with the "messageDigestAlgorithm" and "messageDigest" elements
> empty. The result is the same, but I would prefer to produce valid MIX
> that can be added to later, hence my question.
> Many thanks.
> Ed Fay
> --------------------------------------------
> Developer,Research Fellow     BOPCRIS,EASiHE
> University of Southampton
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Rick Beaubien
Software Engineer, Research and Development
U.C. Berkeley Library

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