A compiler for a minimal subset of Forth, along with an associated VM and visual debugger, all implemented in PostScript.
If you'd like to give this a spin, the easiest way is to install [GhostScript](http://pages.cs.wisc.edu/~ghost/) and then launch the script interactively:
> gs four.ps
You'll be given the option to advance the simulation one "tick" at a time by pressing return. Otherwise, just open the file in the postscript viewer of your choice (including many PDF viewers) and be treated to a "flipbook" of the execution trace.