code: def category_label(arg, labels, nulls=None):
      
      op = CategoryLabel(arg, labels, nulls)
      return op.to_expr()
docs: Format a known number of categories as strings
  
      Parameters
      ----------
      labels : list of string
      nulls : string, optional
        How to label any null values among the categories
  
      Returns
      -------
      string_categories : string value expression