code: def _do_stack(self, cmd, args):
          
          if not args:
              self.__dump_stack()
              return
          if len(args) > 1:
              self.stderr.write('stack: too many arguments: {}\n'.format(args))
              return
          try:
              depth = int(args[0])
          except ValueError:
              self.stderr.write("stack: depth is not an integer: '{}'\n".format(args[0]))
              return
          if depth < 0:
              self.stderr.write('stack: negative depth: {}\n'.format(depth))
              return
          return depth
docs: \
          Manage the shell stack.
              stack               Display the stack.
              stack <depth>       Exit to the stack by its depth.