我有以下幾段代碼：接口及功能定義：package helperimport "gopkg.in/mgo.v2/bson"// Defines an interface for identifiable objectstype Identifiable interface {    GetIdentifier() bson.ObjectId}// Checks if a slice contains a given object with a given bson.ObjectIdfunc IsInSlice(id bson.ObjectId, objects []Identifiable) bool {    for _, single := range objects {        if single.GetIdentifier() == id {            return true        }    }    return false}滿足“可識別”的用戶結構的定義：package modelsimport (    "github.com/my/project/services"    "gopkg.in/mgo.v2/bson")type User struct {    Id       bson.ObjectId   `json:"_id,omitempty" bson:"_id,omitempty"`    Username string          `json:"username" bson:"username"`    Email    string          `json:"email" bson:"email"`    Password string          `json:"password" bson:"password"`    Albums   []bson.ObjectId `json:"albums,omitempty" bson:"albums,omitempty"`    Friends  []bson.ObjectId `json:"friends,omitempty" bson:"friends,omitempty"`}func GetUserById(id string) (err error, user *User) {    dbConnection, dbCollection := services.GetDbConnection("users")    defer dbConnection.Close()    err = dbCollection.Find(bson.M{"_id": bson.ObjectIdHex(id)}).One(&user)    return err, user}func (u *User) GetIdentifier() bson.ObjectId {    return u.Id}檢查切片內對象是否存在的測試：package controllersimport (     "github.com/my/project/helper"    "gopkg.in/mgo.v2/bson")var testerId = bson.NewObjectId()var users = []models.Users{}/* Some code to get users from db */if !helper.IsInSlice(testerId, users) {    t.Fatalf("User is not saved in the database")}當我嘗試編譯測試時，我收到錯誤：undefined helper.IsInSlice. 當我重寫IsInSlice方法以不采取[]Identifiable但[]models.User它工作正常。有任何想法嗎？