The composer of a COMPOSITION should always be known, not just if it happens
to be one with an EVENT_CONTEXT. In the current specification, EVENT_CONTEXT
is only recorded on the one COMPOSITION in a CONTRIBUTION group, which
corresponds to the health system event, like a patient encounter. That means
that there is no composer on the other COMPOSITIONS in the CONTRIBUTION, which
is probably dangerous.

When the composer is the patient, the composer attribute will be set to
a value of PARTY_PROXY which corresponds logically to "self". Details of this
are in the Common IM, generic package.