Thank you for your answer. At the end I found out the way, as you suggested, to get him! For sure I am not particularly brilliant as it took me quite a while to solve the mistery. Luckily I knew of the existence of a Koster who "bred" a Dick Koster and he had to be there somewhere. So I stubbornly fought to the victory! But most of the people are not aware of the difference between a sport and a hybrid so they are not aware of the difference between a breeder and a discoverer. Fortunatly most of the breeder are also discoverer and by searching their name you can have them listed. Then, by clicking the name you can see either the bred or discovered roses. Unfortunatly Koster D.A. is one of the few breeder who was not a breeder but only a discoverer! Then, by checking the breeder button I could not have him listed; as you suggested I had to check the discoverer button to get him! Wouldn' t it be simpler just to have the breeder buttom that let all (breeders and discoverers) be listed and only by clicking the name you find if they bred and/or discovered roses?
The world is full of not particularly brilliant people who love roses ... can you do something for us?