@book { WolframundderGral,
    title = "Wolfram und der Gral",
    subtitle = "Neue Parzival-Studien",
    author = "Samuel {Singer}",
    year = "1939",
    publisher = "Peter Lang Verlag",
    address = "Lausanne, Switzerland",
    url = "https://www.peterlang.com/document/1069261"
}