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Forth is a programming language written in 1969 by Charles Moore, and here, 13 years later came this ZX-81 implementation. Although not in wide use today, it is still being used. A good introduction to Forth can be found on Wikipedia.
Included here are the original cassette and 2 backups and a 60 page manual.
Sorry, no returns. I can't guarantee the tapes will load, given they are 30 years old, but I did listen to one of the backups and it sounded normal, as best I remember they sounded at least. Given the 2 backups, I believe your chances are pretty good.
I will ship to US address by economical Media Mail, unless you need them in a hurry.
If you have any questions, please ask.
Thanks for looking!