Short summary of the proposal:

add a flag "nillable" to each CMDI element (including String, ValueScheme and Pattern) and add a "nillable list" with explicitly declared values in ISOcat (e.g. ​Unknown and ​Unspecified), this will lead to instances with nil-valued elements like:

<startDate xsi:nil="true" cmdi:nilMeaning="unspecified" />
<startDate xsi:nil="true" cmdi:nilMeaning="unknown" />

Advantages:

• using the XML-standard xsi:nil mechanism
• allows for nil-valued validating fields of an XML type that do not allow the empty string (eg boolean or date)
• makes the choice for "nil" fully semantically defined
• leaves the type of the elements at a certain XPath predictable (no <xsd:choice> involved for the element itself, only for the attributes)

As this will use the cmdi: namespace prefix it is suggested to add this to CMDI 1.2

Needed steps:

• changing the component XSD and comp2schema.xsl (Menzo)
  • [Twan] Depending on the way we will implement this, we may run into this: ​http://www.w3.org/TR/xmlschema-1/#cos-nonambig, I'm not sure if there is a way around it but perhaps an XSD expert can look at it.
• adding support to the component registry (Twan)
• adding support to Arbil (Peter/Twan?)

Timeplan:

• create a milestone for the CMDI 1.2 tickets and establish a timing (short/mid-term, Twan will make a milestone definition)