The description of the Concept type is inconsistent between the CQL language definition, and the ELM specification.

1, The language reference [Concept](https://cql.hl7.org/02-authorsguide.html#concept) doesn't pronounce on whether codes and/or display are required, implying they may be empty and/or null.

2. The grammar [Concept](https://cql.hl7.org/19-l-cqlsyntaxdiagrams.html#conceptSelector) requires at least one code in the instance selector and for Concept Definition.

3. The type specification [Concept](https://cql.hl7.org/09-b-cqlreference.html#concept-1) doesn't pronounce on whether codes and/or display are required, implying they may be empty and/or null.

4. The ELM specification [Concept](https://cql.hl7.org/04-logicalspecification.html#concept) follows the grammar and requires at least one code.

5. The ELM specification [ConceptDef](https://cql.hl7.org/04-logicalspecification.html#conceptdef) follows the grammar and requires at least one code.

Given that the Concept type in CQL is intended to be analogous to a FHIR CodeableConcept, this should be clarified by:

1. Making clear in the language reference that codes may be an empty list or a null, and that the display may be null.
2. Relaxing the grammar to allow an empty or null code list to be specified.
3. Making clear in the type specification that codes may be an empty list or a null and that the display may be null.
4. Relaxing the type specification to allow 0..* codes
5. Relaxing the conceptdef specification to allow 0..* codes

In addition, make clear that the term "display" in Concept is a different element than the "display" element of codes in the concept and can be used independently.