They are not necessarily incompatible. Historically, Lisp pairs are mutable, but that doesn't have to be the case. See Clojure, it is Lisp-like, with most data

They are not necessarily incompatible. Historically, Lisp pairs are mutable, but that doesn't have to be the case. See Clojure, it is Lisp-like, with most data structures immutable. Your example is not possible in an immutable list. Also note that this reassigning of variables is a red flag even in standard Lisps. In fact, variables are not that common. The most common way of referencing something for later usage is the let form, which already creates new "variables" anyway. Just eliminate ways to reassign them (setf?). What you would do is to create a new list containing the elements you want. You may or may not want to give it a new name afterwards. Most likely, you'll use as it is and pass on to some other function. I can't comment on this particular implementation. But it does not necessarily have to be a bottleneck. In fact, by having immutable data structures, that means that you can share data like crazy, without fear of anything being replaced where it should not. All sorts of optimizations are enabled that wouldn't be possible otherwise. If this implementation uses them, it's another matter entirely. No idea.