[README] Example 02

Here we have the first non-trivial example of a styx application.
It is a somewhat complete little programming language approximately
of the complexety of LISP.

The example demonstrates to use of the derivation tree as a
source representation beyond parsing. Here, we use it to keep
the definition of functions available for execution.

Additionally, full static and dynamic semantics of the language
is implemented to introduce the use of the "Map" and "symbol"
data type together with other handy library routines as the
tree iterator and the PT_error routine.

An (atypical) use of the pretty printing abilities is also
provided.

The profiling webed into the example gives an impression of
the efficiency of the whole library material. Note that the
interpreter is not optimized for speed.

With a total of about 250 lines of C code and 100 lines for
the grammar, which took about 4 hours to be written from
scratch including debugging, this example also shows efficient
a compiler/interpreter author can be with styx.

Use the shell script 'mx' to create the executable. 
Don't forget 
1) to check the setting of the environment variable STYXDIR in 'mx'
2) to add $STYXDIR/lib to your library path.

Execute the example program [testpl0.pl0] by 'pl0 testpl0.pl0'
or adjust the path in the first line of [testpl0.pl0], set the
executable flag and call it directly.