Add a property to FamilyMemberHistory.condition to indicate that a family member did not have the condition (e.g., mother's brother did not have hemophilia).

To align with the coded Observation pattern that supports pertinent negatives (#3), use an explicit coded value. Do not use the abstract and ambiguous Boolean type to represent the concept of absence. Support values for present and absent, with present being defined as the default for instances not containing the element. This is a modifying element. Compare FHIR-38899.