From a pool of of candidate Deliverable Units, determine which - if any - of the Deliverable Units is
the root of an object graph.
Because the User Interface maps business objects into a graph headed by a single Deliverable Unit,
and because UI objects are deposited one at a time into the DCS, a deposit into the archive is
guaranteed to have a single Deliverable Unit as the root of the object graph (though the graph may contain
descendant Deliverable Units as child nodes in the graph).
Given all of the Deliverable Units that comprise a deposit, determine which Deliverable Unit is the root
of the object graph. If no root can be determined, or if there are multiple possible roots,
nullis returned.