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Marbelous post-dates this question, but since I made recursive fib(n) the canonical example of the language in the docs for my interpreter, I thought it worth posting:
:Fb
}0 }0 }0 .. # three copies of the first input, call them A B C
-- &0 >1 {0 # decrement A, hold B, return C if it's <2
&0 -- >...