;; (FIB TARGET) computes the element of the Fibonacci sequence at TARGET (zero-indexed).
(letrec ((next (lambda (a b n target)
                 (if (eq n target)
                     a
                     (next b
                           (+ a b)
                           (+ 1 n)
                           target))))
         (fib (next 0 1 0)))
        (fib 5))