public CodeElement Item([NotNull] object index)
 {
     if (!(index is int number))
     {
         throw new ArgumentException();
     }
     return(Registrar.Convert(CodeElementModels[number], Parent));
 }
 public IEnumerator GetEnumerator() => CodeElementModels
 .SelectNotNull(model => Registrar.Convert(model, Parent))
 .GetEnumerator();