XMI12 — MOF structured types missing from XMI 1.2

In keeping with the new MOF 1.4 specification, XMI 1.2 has removed all
references to the "old" MOF type mechanism based on CORBA IDL types. A
wonderful simplification of both MOF and XMI - no complaints!

However, the new simplified MOF type concept has not been included.
Specifically: there are no explicitly stated rules for encoding
StructureType, AliasType or CollectionType.

Probably linked to this is the fact that the pre-defined XMI elements
XMI.field, XMI.sequence and XMI.seqItem are stated to be "...used only when
required by the metamodel" [XMI 1.2, pp. 4-17]. And since they are never
referenced by the rest of the specification, this apparently means "never".

It is of course not difficult to connect these two peculiarities and deduce
how the new MOF datatypes should be encoded, but it should really be stated
explicitly.