@book { DasfreieSpiel,
    title = "Das freie Spiel",
    author = "Diana Gabriela {Födinger}",
    year = "2008",
    publisher = "Diplom.de",
    address = "Hamburg, Deutschland",
    doi = "10.3239/9783836620970",
    url = "https://m.diplom.de/document/226272"
}