code: func (c *Clique) VerifyUncles(chain consensus.ChainReader, block *types.Block) error {
  	if len(block.Uncles()) > 0 {
  		return errors.New("uncles not allowed")
  	}
  	return nil
  }
docs: // VerifyUncles implements consensus.Engine, always returning an error for any
  // uncles as this consensus mechanism doesn't permit uncles.