Fix the slicing of AllergyIntolerance.code to discriminate on the value set binding (type=value, path=$this). Define the slice AllergyIntolerance.code:absentOrUnknownAllergy with a required binding to http://hl7.org/fhir/uv/ips/ValueSet/absent-or-unknown-allergies-uv-ips (the AllergyIntolerance.code.coding and AllergyIntolerance.code.coding.system elements are not constrained and therefore are not included in the slice definition). This allows explicity representing the absentOrUnknownAllergy case including the value set binding and declaring the slice mustSupport and rendering all of that in the differential. The previous max=1 contstraint on AllergyIntolerance.code.coding in the absentOrUnknownAllergy slice has been removed, which allows code translations (if there's ever any need for that) as is normally expected for a CodeableConcept.