>>16 That's very clever. It seems to lose accuracy around fib(72). More accuracy means adding more accuracy to PI, and that means you'd need a constant-time function to find bits of PI.

I do notice that the CL version has fewer "parens" (braces, semicolons, etc) than the C version >>6 wrote.

(defun fib (n)
 (loop repeat n
  with p = 0 and q = 1
  do (psetq p q q (+ p q))
  finally (return p)))