Added example to chapter7

This approach reduces the number of stack arguments to \forth{BOX} as

part of the design.

\begin{interview}

\person{Bernd Paysan}:

\begin{tfquot}

I prefer turtle graphics. This is a turtle graphics without cursor

direction, so the direction is explicit (using the same \forth{LINE}

primitive):

\begin{Code}

2variable pos

: setpos ( x y -- )  pos 2! ;

: p+ ( x y w h -- x' y' )  rot + >r + r> ;

: moveto ( dx dy -- )  pos 2@ p+ pos 2! ;

: drawto ( dx dy -- )  pos 2@ 2swap moveto pos 2@ line ;

: right ( dx -- )  0 drawto ;

: down  ( dy -- )  0 swap drawto ;

: left  ( dx -- )  negate right ;

: up    ( dy -- )  negate down ;

: box ( w h -- )  2dup down right up left ;

: [box] ( x y w h -- )  2swap setpos box ;

\end{Code}

Now this seems to be a lot of work for a single box, to get rid of the

long definition and the stack juggling. But usually, it doesn't end

with a single box. And then the small initial work pays off big.

I think this is a good illustration how \Forth{} code should look

like. None of the definitions has more than 7 words. Only one has more

than one stack juggling word. The global variable now is no disguised

``local,'' but the state (position) of the turtle.

\end{tfquot}

\end{interview}

\begin{tip}

When determining which arguments to handle via data structures rather

than via the stack, choose the arguments that are the more permanent or