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This is proof by contradiction, where we assume the opposite of what we want to prove and show that it is logically inconsistent.

If we call (try try) then we get the following (with try substituted in for p)

```
(define (try try)
(if (halts? try try)
(run-forever)
'halted))
```

We observe that if (halts? try try) is True then (try try) will run forever, which is a contradiction. The converse, if (halts? try try) if False then (try try) returns 'halted, could also be used to reach a contradiction.

meteorgan