code: def _variable_or_iterable_to_set(x):
      """
      Convert variable, set, or iterable x to a frozenset.
  
      If x is None, returns the empty set.
  
      Parameters
      ---------
      x : None, str or Iterable[str]
  
      Returns
      -------
      frozenset : frozenset representation of string or iterable input
      """
      if x is None:
          return frozenset([])
  
      if isinstance(x, str):
          return frozenset([x])
  
      if not isinstance(x, Iterable) or not all(
          isinstance(xx, str) for xx in x
      ):
          raise ValueError(
              f"{x} is expected to be either a string, set of strings, or an iterable of strings"
          )
  
      return frozenset(x)
  
label_desc: Use image node_id 1 for calling a global function with example usage: _variable_or_iterable_to_set(x) and returns: frozenset, frozenset, frozenset