code: def new(arg_name, annotated_with=None):
      
      if annotated_with is not None:
          annotation = annotations.Annotation(annotated_with)
      else:
          annotation = annotations.NO_ANNOTATION
      return BindingKey(arg_name, annotation)
docs: Creates a BindingKey.
  
      Args:
        arg_name: the name of the bound arg
        annotation: an Annotation, or None to create an unannotated binding key
      Returns:
        a new BindingKey