     // Singleton
     // http://stackoverflow.com/questions/35591466/create-singleton-instance-via-extension-per-app
     //
-    open static let shared = SwiftyHealthKit()
+    public static let shared = SwiftyHealthKit()
     private init() {}
     public let store = HKHealthStore()
     

    }
    
    public func overwriteSample(at date: Date, id: HKQuantityTypeIdentifier, quantity: HKQuantity, metadata: [String: String]? = nil, completion: @escaping Callback<Bool>) {
        
        deleteData(at: date, id: id) { result in
            switch result {
            case .failure(let error): completion(Result.failure(error))

    }
    
    private func statistics(at date: Date, id: HKQuantityTypeIdentifier, option: HKStatisticsOptions, completion: @escaping Callback<HKStatistics?>) {
        statisticsCollection(at: date, id: id, option: option) { result in
            switch result {
            case .failure(let error): completion(Result.failure(error))
            case .success(let collection):
                guard let collection = collection else {
                    completion(Result.success(nil))
                    return
