We will split the event into eventCode and eventReference and add an invariant that at least one must be present. Will add documentation that either or both can be present. If both are present, then the eventCode indicates an event that occurs within the context of the reference. Will provide examples for: offset from date of birth (code only) (after end of) offset from Procedure (event only) (before start of) offset from start of treatment within a clinical trial (Reference to ResearchStudy, eventCode of "start of treatment") (after end of)