Issue 12055 Map referencing behaviors

In an attempt to draw out more responses on this one ...

The issue under discussion is how to define differences in behaviors. To
make it easier, I will give a specific example.

We have a chapter element defined in bookmap. When this points to a map, it
casts the referenced material in the role of a chapter, or a sequence of
chapters if there are multiple top-level elements. Attributes and metadata
will cascade to the referenced 'chapters'.

Within IBM we have another specialization,