added preface for the ANS edition

preface-ans.tex

+\chapter*{Preface to the ANS \Forth{} Edition}\label{preface-ans}
+\pagestyle{headings}
+
+\initial The original 1984 \emph{Thinking \Forth} feels a bit dated
+today. A lot happend with \Forth{} in the last 20 years, since this
+book was first published. One of the most important is the ANS
+\Forth{} standard from 1994. Unlike previous \Forth{} standards, it
+provided necessary abstraction for machine word size and compilation
+methods. Few \Forth{}s today are still indirect threaded code, and
+even fewer are 16 bit systems.
+
+What changed, too, is coding style. \Forth{} programs are rarely
+written all uppercase these days. Like other languages that started
+with uppercase keywords, the result are case insensitive systems---at
+least for the ASCII subset of the character set.
+
+Screens are no longer the dominant way to keep sources. Forth
+development systems usually are hosted on a modern (large) operating
+system, and most people keep their sources in files.
+
+Operating systems now provide services to programs that weren't
+possible 20 years ago, and modern \Forth{} systems must be able to use
+them. Paradigms like object oriented programming were adopted to
+\Forth{}.
+
+All these changes demand a rewrite of this book. Since \person{Leo
+Brodie} released the original under a Creative Commons license, this
+is now possible. This edition adds all the missing things from the
+original:
+
+\begin{itemize}
+\item Modify the example sources so that they run with ANS \Forth{}
+systems.
+
+\item Update coding style to current practice (lower case and such).
+
+\item Add chapters about \Forth{} and OOP, \Forth{} debugging, and
+maintenance.
+
+\item Interview \Forth{} thinkers that didn't have a chance 20 years
+ago.
+\end{itemize}
+
+\begin{flushright}
+\vspace{5em}
+\person{Bernd Paysan}
+\vspace{2.5em}
+\end{flushright}
+