docs: def doc_string(cls):
          """Get the doc string of this class.
  
          If this class does not have a doc string or the doc string is empty, try
          its base classes until the root base class, _ShellBase, is reached.
  
          CAVEAT:
              This method assumes that this class and all its super classes are
              derived from _ShellBase or object.
          """
          clz = cls
          while not clz.__doc__:
              clz = clz.__bases__[0]
          return clz.__doc__