Need to be able to point to the order, proposal, recommendation, etc. that caused the Observation to occur. Should probably be a bit broader, so perhaps allow it to point to CarePlan, ProcedureRequest, ReferralRequest and DiagnosticOrder. Having a link on DiagnosticReport is not sufficient as DiagnosticReports might not always exist and there's no context conduction between DiagnosticReport and the Observations it contains.