remarks and suggestions greatly improved the book. I am very thankful
to Neelesh Tiruviluamala for pointing out a mistake in an earlier draft.
I am also grateful to Bernard Lascar and Ciprian Manolescu for valuable
comments. I am especially grateful to my wife Marina who prepared the
manuscript. Without her help this book would not be written.
My deepest gratitude to the editor Natalya Pluzhnikov for the outstand-
ing work of improving and correcting the manuscript. It is impossible to
overestimate her contribution.
Finally, I am very grateful to Sergei Gelfand for encouragement to write