I've not really encountered any situations where I've needed to do anything like this yet (still learning), but, I found this example, where there's an existing function, and someone alters the body of the function to create a new function. My guess is that there's various ways of doing this type of stuff: https://stackoverflow.com/a/1220198

Edit: a macro example, which I think may be the more common route of doing this type of stuff? https://lisp-journey.gitlab.io/blog/common-lisp-macros-by-ex...