docs: def bool_set(operation,*sets):
      '''combine ``sets`` with ``operation``
      
      :and:       combine sets with AND
      :or:        combine sets with OR
      :not:       NOT sets versus the last given set'''
      if len(sets)<=1:
          return sets
      if operation=='and':
          return and_sets(*sets)
      if operation=='or':
          return or_sets(*sets)
      if operation=='not':
          return not_sets(*sets)