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Firebase Admin SDK in use: https://firebase.google.com/docs/reference/admin/node/admin.messaging
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The Firebase API in use: https://firebase.google.com/docs/reference/fcm/rest/v1/projects.messages
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Firebase Errors: https://firebase.google.com/docs/reference/fcm/rest/v1/ErrorCode
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Convert APN tokens to FCM: https://www.thepolyglotdeveloper.com/2017/06/apns-tokens-fcm-tokens-simple-http/