https://books.google.es/books?id=KPZTCgAAQBAJ&pg=PA87&lpg=PA87&dq=erich+graichen&source=bl&ots=paNSIKD1nP&sig=3_7awxI8zq8Qugb9zK79BaHAmko&hl=es&sa=X&ved=0ahUKEwie2NGvyenVAhVBa1AKHWN0Aq4Q6AEITTAN#v=onepage&q=erich%20graichen&f=false
