@book { DieHarmonisierungderindirektenSteuerninEuropa,
    title = "Die Harmonisierung der indirekten Steuern in Europa",
    author = "Alexander {Hagen}",
    year = "2021",
    publisher = "Peter Lang Verlag",
    address = "Berlin, Germany",
    url = "https://www.peterlang.com/document/1090714"
}