Dijkstra's example of an ambiguous program [closed]

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Salutations. Dijkstra wrote that even a 开发者_如何学运维few lines of seemingly simple code could be hopelessly ambiguous. In at least one work, which I can't find now to save my life, he gave a little example program to demonstrate this ambiguity. Can anybody point me to a paper of his where he includes one of these examples?