: interior and exterior. 

<P>
<DL>
<DT><STRONG>Interior Properties</STRONG></DT>
<DD>are stored ``inside'' the graph object
in some way, and the lifetime of the property value objects is the
same as that of the graph object. 

<P>
</DD>
<DT><STRONG>Exterior Properties</STRONG></DT>
<DD>are stored ``outside'' of the graph
object and the lifetime of the property value objects is independent
of the graph. This is useful for properties that are only needed
temporarily, perhaps for the duration of a particular 