docs: def unscan(self):
          """
          Undo the last scan, resetting the position and match registers.
  
              >>> s = Scanner('test string')
              >>> s.pos
              0
              >>> s.skip(r'te')
              2
              >>> s.rest
              'st string'
              >>> s.unscan()
              >>> s.pos
              0
              >>> s.rest
              'test string'
          """
          self.pos_history.pop()
          self._pos = self.pos_history[-1]
          self.match_history.pop()
          self._match = self.match_history[-1]