The specification is unclear about the expected runtime semantics of interval and list-valued operators, including query.

Propose that for any operation, if any runtime value is list-valued, the operation is the list overload and should return a list. If all runtime values are null, the result is null (which is compatible with list-valued and interval-valued operations).

Specifically, clarify for query evaluation semantics that: if any source evaluates to a list, the result is list-valued, if all sources evaluate to null, the result is null

Clarify for union: if any source evaluates to a list, the result is list-valued, if all sources evaluate to null, the result is null

Here is an example library that illustrates expected behavior for query cardinality:

library NullSourceQueries version '0.0.1'using FHIR version '4.0.1'include FHIRHelpers version '4.1.000' called FHIRHelperscontext Patient// The query below (over a null list) should probably return either
// a) null, or
// b) an empty list
// Resolution: Return null when the source is null
define NullListQuery:
  (null as List<FHIR.CodeableConcept>) X
    return Concept {
      codes: X.coding C return FHIRHelpers.ToCode(C)
    }// The query below (over a null object) should probably return either
// a) null, or
// b) a Concept w/ empty (or null) codes
// Resolution: Return null when the source is null
define NullObjectQuery:
  (null as FHIR.CodeableConcept) X
    return Concept {
      codes: X.coding C return FHIRHelpers.ToCode(C)
    }