We hope that this makes the book inconvinient to glide over. The core material is enhanced by a lot of problems of various sorts and additional pieces of theory. This enhancements can be recognized by wider margins, as the next paragraph. The problems, which do not comprise separate topics and are intended exclusively to be exercises, are typeset with small face. Some of them are very easy and included just to provide additional examples.
Few problems are difficult.
They are to indicate relations with other parts of mathematics, show possible directions of development of the subject, or just satisfy an ambitious reader. Maybe, a mathematician, who does not work in the fields geometric in flavor, can afford the luxury not to know some of these things. Maybe, students studying topology can postpone this material to their graduate study. We would like to include this to graduate lecture courses. However, quite often it does not happen, because most of the topics of this sort are rather isolated from the contents of traditional graduate courses.