extension_ property
extension_ ("extension") May be used to represent additional information
that is not part of the basic definition of the element. To make the use
of extensions safe and managable, there is a strict set of governance
applied to the definition and use of extensions. Though any implementer
can define an extension, there is a set of requirements that SHALL be met
as part of the definition of the extension.
Implementation
@JsonKey(name: 'extension')
List<FhirExtension>? get extension_ => throw _privateConstructorUsedError;