I don't have a simple example (and I'm not claiming [some] loops do not have sensible derivatives). This talk from 27mins onwards discusses some of the issues

I don't have a simple example (and I'm not claiming [some] loops do not have sensible derivatives). This talk from 27mins onwards discusses some of the issues if you're interested in diving in a bit deeper: https://www.youtube.com/watch?v=qhPBfysSYI8 https://www.youtube.com/watch?v=qhPBfysSYI8 I found it pretty heavy going. YMMV.