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First, we will ammend the schema to support the property "optional" to datastream declarations. The JIRA issue for this is http://fedora-commons.org/jira/browse/FCREPO-531
The semantic meaning of optional is:
1. The datastream can exist in the substribing objects but does not have to
2. If the datastream exist, it must adhere to the specification in the content model
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Notice that in the import tag, the schema location is given as "$THIS$/SIMPLEDC-SCHEMA". This means that the schema location is the residing in the SIMPLEDC-SCHEMA datastream in this content model. This functionality only works in schemas, and only when validating with the built-in validate method. It works both for schemas inlined in DS-COMPOSITE-MODEL, and stored in separate datastreams. You can have multiple schemas imported, in a hirachical structure, but the $THIS$ will always refer to the object containing the DS-COMPOSITE-MODEL in question.
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