The type of references used from EHR to CONTRIBUTION
should not be a value reference, since most of the time, previous
CONTRIBUTIONs are not of interest. When an EHR object is retrieved
from a system, usually only the current DIRECTORY/FOLDER structure
should be retrieved with it. Otherwise, all CONTRIBUTIONs and
COMPOSITIONs should be left until explicitly retrieved according
to particular query scenarios.