func firebaseSignInWithLink(credential: FIRAuthCredential) {

        FIRAuth.auth()?.signIn(with: credential, completion: { (user, error) in
            if error != nil {
                debugPrint("APP: there has been an error signing into firebase, perhaps another account with same email")
                debugPrint("APP: (error)")

                // if existing email, try linking
                FIRAuth.auth()?.currentUser?.link(with: credential, completion: { (user, error) in
                    if error != nil {
                        debugPrint("APP: there has been an error signing into firebase")
                        debugPrint("APP: (error)")
                    }
                    else {
                        debugPrint("APP: successfully signed into firebase")
                    }
                })
            }
            else {
                debugPrint("APP: successfully signed into firebase")
            }
        })
    }