[README] Example 03

This example is a quick variation of the interpreter in Example 02.
Use the shell script 'mx' to create the executable. Don't forget to check
the setting of the environment variable STYXDIR in 'mx'.

It demonstrates persistence as a feature of Styx.
All the modification wrt. Example 02 is to split
the [pl0.c] program apart into two parts:

1) A "compiler" [pl0c.c], which parses the source,
   does the static semantics, stores the derivation
   tree into a file.

2) A "run time system" [pl0r.c] which reads and
   executes the so-produced binary image.

"compile" [testpl0.pl0] by 'pl0c testpl0' yielding [testpl0].

If you browse the file, you find it starting with
something like "#!/p/ping/pl0r". You may want to
adjust this path issued in [pl0c.c] to the location
of the pl0r binary and do a 'chmod +x testpl0' for
a real executable.

Otherwise run it using 'pl0r testpl0'.

One may argue, that this is not a "real" compiler,
which should create pseudo code, at least. This is
true, but writing an intepreter for pseudo code that
is significantly faster then this example is not so
trivial as one might think.

Perhaps we will continue later with an example of a 
proper to-pseudo-code compiler and a nice little machine, 
but this may never be necessary, since there is something 
as strong as Styx itself on top of it, which may soon be 
ready for prime time.