code: def goBack(self):
          
          if not self.canGoBack():
              return ''
          
          self._blockStack = True
          self._index -= 1
          self.emitCurrentChanged()
          self._blockStack = False
          return self.currentUrl()
docs: Goes up one level if possible and returns the url at the current level.
          If it cannot go up, then a blank string will be returned.
          
          :return     <str>