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. [http://www.isocat.org/datcat/DC-2591 Unknown] and [http://www.isocat.org/datcat/DC-2592 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 [source:metadata/trunk/toolkit/general-component-schema.xsdgeneral component XSD] and [source:metadata/trunk/toolkit/xslt/comp2schema-v2/comp2schema.xsl 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)