let
  pkgs = import <nixpkgs> {};
  tracer = n: builtins.trace "tracing: ${pkgs.lib.generators.toPretty {} n}" n;
in with import ./lisp.nix { a = null; }; (eval
(defun fib (n)
  (defun fib2 (a b n)
    (iff (lt n 2) b
      (def (next (plus a b))
      (fib2 b next (minus (lift tracer n) 1))))
  (fib2 0 1 n))
(fib 12)))