
Sentinel Value - tosh
https://en.wikipedia.org/wiki/Sentinel_value
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zapf

           1. Go to Africa.
           2. Put an elephant in Cairo.
           3. Start at the Cape of Good Hope.
           4. Work northward in an orderly manner, traversing the continent alternately east and west,
           5. During each traverse pass:
    
                   1. Catch each animal seen.
                   2. Compare each animal caught to a known elephant.
                   3. Stop when a match is detected.
    
            6. If you are in Cairo, then there are no elephants in Africa (other than the one you placed there).

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dragonwriter
This approach doesn't work.

The sentinel elephant you place has to be closer to Algiers than Cairo, or you
risk false negatives.

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zapf
TIL: A sentinel value is sometimes known as an "Elephant in Cairo", due to a
joke where this is used as a physical sentinel.

[https://en.wikipedia.org/wiki/Elephant_in_Cairo](https://en.wikipedia.org/wiki/Elephant_in_Cairo)

