Define semantics for occurrences on ARCHETYPE_INTERNAL_REF

In the current version of the ADL and AOM specifications, th semantics of the
occurrences for nodes of type ARCHETYPE_INTERNAL_REF (the node type in an
archetype that allows a node to refer to a previously defined node for its
definition) are not properly defined.

The need is for occurrences at the reference node to be able to override the
occurrences of the target node, or if not mentioned to be the same.