  These notes are based on a lecture course by L. Chekhov held at the
University of Manchester in May 2006 and February-March 2007. They are
divulgative in character, and instead of containing rigorous mathematical
proofs, they illustrate statements giving an intuitive insight. We
intentionally remove most bibliographic references from the body of the text
devoting a special section to the history of the subject at the end.
