I first learned of it in Douglas Hofstadter’s Metamagical Themas, I think. You can run it here: https://archive.org/details/msdos_Racter_1984 https://archive.o

I first learned of it in Douglas Hofstadter’s Metamagical Themas, I think. You can run it here: https://archive.org/details/msdos_Racter_1984 https://archive.org/details/msdos_Racter_1984