The invariant documented on Immunization.education appears to be too general.  It appears to imply that Immunization.education must always be present in Immunization, even though the cardinality of Immunization.education is 0..*.  We suggest that the invariant be updated to include “If Immunization.education is present, then…..”.  For abundance of clarity to avoid misinterpretation, and even if this is documented as a general way to describe invariants that span elements of a BackboneElemnet, we believe clarity is critical that it not interpreted at the attribute level.