$ erl
Erlang R14B02 (erts-5.8.3) [source] [smp:2:2] [rq:2] [async-threads:0] [hipe] [kernel-poll:false]

Eshell V5.8.3  (abort with ^G)
1> c(onion).
{ok,onion}
2> onion:run(5).
5
3> onion:run(10).
55
4> 

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onion:run(N) prints Nth Fibonacci number, but it can do much more...

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What this is:
(added on 2011/08/14)

onion.erl has an interpreter for a stack-based language inside of it, as well as a program in that language that calculates the Nth Fibonacci number. Several features of Erlang are used to obfuscate the code: parse_transform, undefined_function, dynamic compilation of modules, and function names as any atoms.