Currently the EHR IM defined COMPOSITION.content as of type SECTION.
However, it would be more flexible to be of type CONTENT_ITEM, since
it would allow for COMPOSITIONS which contained ENTRYs directly,
and particularly COMPOSITIONs which contained GENERIC_ENTRYs directly.
Archteypes can safely control which kind of class is actually used
in any given circumstance.