It's not clear whether the meaning of a Patient with no "animal" element is "Patient is a human" or "It's unknown whether the patient is a human or an animal" (meaning that if you want to force it to be a human, you must declare animal with species of homo sapiens in some code system or other). That meaning must be clear and be consistent across all systems. This can be done by defining the "meaning when missing" property. If the decision is "Patient.animal absent = human", then there should also be guidance about what to do for systems that don't necessarily know if they're dealing with humans or animals.