@book { TropfenaufkaltenStein,
    title = "Tropfen auf kalten Stein",
    author = "Ulla {Schneider}",
    year = "2012",
    publisher = "hey! publishing",
    address = "München, Deutschland",
    doi = "10.3239/9783942822138",
    url = "https://www.heypublishing.com/document/206220"
}