A Dependable is an in-memory representation of an object managed
by the Dependency System.
There are two kinds of Dependables:
Providers and Dependents. Dependents depend on Providers and
are responsible for executing compensating logic when their
Providers change.
Get an object which can be written to disk and which,
when read from disk, will find or reconstruct this in-memory
Dependable.
A Finder object that can be written to disk if this is aPersistent Dependable.Null if this is not a persistent dependable.