This is a web-based interpreter for Tandem, a rewriting language where programs form a Kleene algebra.
Technically, it is Language.Tandem (the reference implementation
of Tandem, written in Haskell) compiled to JavaScript
by the Haste compiler, running in an HTML5 document.
Instructions:
Select an example program from the dropdown box and click "Run" to run it. You can also edit the program before running it. Or you could clear the textbox and write an entirely new program there and run it.
You might also want to read the description of the Tandem programming language.