Many things in our science and techniques are secret, a whole lot of forbidden themes exist (…). Does this mean that we don't have to show people things which are at the limit of secrecy? No, in each question, even in the most complicated one, we can find the right way that the reader can feel how a task is solved, we can avoid vulgarization, can show how an idea is born. An idea never can be secret, even if the result of an idea is kept secret. (Boris Liapunov, 1951) 1