A C model region describes a hierarchical set of elements.
Regions are often used to describe a set of elements to be considered
when performing operations; for example, the set of elements to be
considered during a search. A region may include elements from different
When an element is included in a region, all of its children
are considered to be included. Children of an included element
cannot be selectively excluded.
Adds the given element and all of its descendents to this region.
If the specified element is already included, or one of its
ancestors is already included, this has no effect. If the element
being added is an ancestor of an element already contained in this
region, the ancestor subsumes the descendent.
Removes the specified element from the region and returns
true if successful, false if the remove
fails. If an ancestor of the given element is included, the
remove fails (in other words, it is not possible to selectively
exclude descendants of included ancestors).