XMI21 — Mandatory Extensions when enforceMinimumMultiplicity is true

My base document is the MOF 2 XMI Final Adopted Specification,
ptc/03-11-04.

There is an issue with rule 4c, part of the XML Schema Production rules,
on page 46.

If either org.omg.xmi.enforceMinimumMultiplicity or org.omg.xmi.enforce-
MaximumMultiplicity is true, then rule 4 places the 4b:ClassContents in
an xsd:sequence element – and subelements are then decorated with
minOccurs and maxOccurs attributes.

However, rule 4c still emits the constant element,
<xsd:element ref='xmi:extension'/>

Therefore, in this case, the extension element becomes mandatory, with
an implicit multiplicity of 1..1. This needs to be changed to
<xsd:element ref='xmi:extension' minOccurs='0' maxOccurs='unbounded'/>