docs: def update_arg(self, arg, if_existent=None, **kwargs):
          """
          Update the `add_argument` data for the given parameter
  
          Parameters
          ----------
          arg: str
              The name of the function argument
          if_existent: bool or None
              If True, the argument is updated. If None (default), the argument
              is only updated, if it exists. Otherwise, if False, the given
              ``**kwargs`` are only used if the argument is not yet existing
          ``**kwargs``
              The keyword arguments any parameter for the
              :meth:`argparse.ArgumentParser.add_argument` method
          """
          if if_existent or (if_existent is None and
                             arg in self.unfinished_arguments):
              self.unfinished_arguments[arg].update(kwargs)
          elif not if_existent and if_existent is not None:
              self.unfinished_arguments.setdefault(arg, kwargs)