public override JsonNode this[int index] =>
 XHelper.Create(elementType, elementCode, values[index]);
 internal XImmutableArray(T[] values)
 {
     this.values      = values ?? throw new ArgumentNullException(nameof(values));
     this.elementCode = System.Type.GetTypeCode(typeof(T));
     this.elementType = XHelper.GetElementType(this.elementCode);
 }