// füge zwei hinzu
server.buyArticle(4, 2);
assertEquals(server.getArticleReserved(4), 12);
// entferne zwei
server.buyArticle(4, -2);
assertEquals(server.getArticleReserved(4), 10);
int sumafter = server.getArticleReserved(4) + server.getArticleAvailable(4);
assertEquals (sumbefore, sumafter);