# [Fortress 0.1 Alpha](http://web.archive.org/web/20070715233437/http://fortress.sunsource.net/files/documents/81/140/fortress_168.zip), 39 bytes.

    f(n)=if n<3then n else f(n-2)-f(n-1)end

This language is old and dead, very dead. (1-indexed: f(1)=1, f(2)=2).

Can be placed into a program like this:

    export Executable
    f(n)=if n<3then n else f(n-2)-f(n-1)end
    run(args) = do
        println(f(3))
    end

Does this **really** need explaining? This is just the definition.