Hi Оля It helps if one is prepared to start simple, and develop complexity after. The lessons which invite you to be corrected constantly, are not nearly as helpful as those which encourage you to take a few bold efforts. I encourage students to read and study those things which have already been written, rather than try to RE-invent the Wheel. The wheel has already been invented you know. Thus, you could ride in the wagon, let the horse do the pulling, because the horse has travelled down the road before. You just need to read a few basic ideas, and start from there. --Bruce