def nth_fib(n):
# input: [f(i-2), f(i-1), countdown]
def fib: (.[0] + .[1]) as $sum
| .[2] as $n
| if ($n <= 0) then $sum
else [ .[1], $sum, $n - 1 ]
| fib end;
[-1, 1, n] | fib;