code: func (e *GroupExpr) FingerPrint() string {
  	if e.selfFingerprint == "" {
  		e.selfFingerprint = fmt.Sprintf("%v", e.ExprNode.ID())
  		for i := range e.Children {
  			e.selfFingerprint += e.Children[i].FingerPrint()
  		}
  	}
  	return e.selfFingerprint
  }
docs: // FingerPrint gets the unique fingerprint of the Group expression.