code: def union(*sets, **kwargs):
      
      sets = _set_preprocess(sets, **kwargs)
      return as_index( _set_concatenate(sets), axis=0, base=True).unique
docs: all unique items which occur in any one of the sets
  
      Parameters
      ----------
      sets : tuple of indexable objects
  
      Returns
      -------
      union of all items in all sets