11 original. Modifications which I introduced are radical and often far
going. In my opinion the Polish text had two main drawbacks. It was
overloaded with informal considerations and at the same time formal
concepts included in some parts of the book were presented in a too
complicated way. Of course one of the motives to revise it was also the
fact that much time has passed since I finished writing the Polish
version and obviously certain decisions and ideas contained in the first
edition seem not quite relevant now. So it is not only the desire to
make the exposition clearer but also the reasons of substantial nature
which motivated writing a revised version. I do not think it desirable
to bother the reader with a detailed discussion of all changes to which
the Polish version was subjected and that is why I will confine myself
to pointing out only the most significant ones. Explanations concerning
logical and set-theoretical notions applied in the book have been
shortened as much as possible, in the Polish version one whole chapter
was devoted to the discussion of them.