>> Has it been proven that the ability to write undecidable programs confers expressive power such that you can't have one without the other? Yes. Can you pro

>> Has it been proven that the ability to write undecidable programs confers expressive power such that you can't have one without the other? Yes. Can you provide a reference? >> It seems self-evident that running forever can't be useful Really? Do you not think that it might be useful for, say, an operating system to run forever? It's nice when the OS has the option of shutting down deterministically.