If I absolutely had to write a recursive solution, something like this (should have all recursive calls in a tail position, may require trading debugability for

If I absolutely had to write a recursive solution, something like this (should have all recursive calls in a tail position, may require trading debugability for speed): (defun tail-fib (n &optional (v0 0) (v1 1) (cond ((zerop n) v0) (t (tail-fib (1- n) v1 (+ v0 v1)))))