See Explain one liner for fibonacci for the exact same question and answers.

For reference, the original source for that one-liner is Re^2: The Oldest Plays the Piano.

Update: As that thread also links to the explanation, I wonder where you found this one-liner but not the explanation. Maybe I've posted this node anonymously while sleepwalking to popularize my obfu?

In reply to Re: Explain Fibonacci one-liner ?? by ambrus
in thread Explain Fibonacci one-liner ?? by Anonymous Monk

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