Heh, that thread brings to mind a bunch of things... One advantage of CL that helps with a 'borrowing' aspect that give people the jeebies is that the unit of compilation is much smaller than the file, so you can also borrow much less. Another is that methods are decoupled from classes so there's a lot of room for extensibility. (The LLGPL interestingly provides an incentive for the open-closed principle, i.e. you extend objects you're fine, but if you modify the library's then you are subject to the LGPL.) If you haven't read Gabriel's Patterns of Software book (free pdf on his site), I think you'd enjoy it.

Your edits won't get blown away, at least by default, since quicklisp doesn't redownload a system that it knows about or check for 'corruption'. The way quicklisp does its own versioning also means if you update quicklisp's distributions lists and a newer version of the library has come out, it'll download that one into its own new folder and leave the old one alone. There's a cleanup function to clear out old things but I don't know of a case where that gets called hidden from you under the hood.

Maybe there's some magic and interesting stuff related to this for emacs or mezanno OS or in the annals of ancient lisp machines but I'm a vim heretic ;) But in any case if you want to save stuff, you can just save it in a new or existing buffer... So options are basically as I described. To give a specific example, I have a script that fetches and parses some HTML to archive HN comments and I wanted to remove the HTML bits so I'd just have text, making it more markdown-y. There are lots of ways to do that and I'm pretty sure I chose one of the worst ones, but whatever. I was already using the Plump library, and after not achieving full success with its official extension mechanisms, one method I came across and stepped through was https://github.com/Shinmera/plump/blob/master/dom.lisp#L428 and I learned I could hijack it for my purposes. I started by editing and redefining it in place until I got what I wanted, but instead of saving my changes over the original, I simply copied it over to my script file, modifying slightly to account for namespaces e.g. it's "(defmethod plump-dom:text ((node plump:nesting-node))", thus redefining and overwriting the library implementation when my script is loaded and run.

Some possible problems with this approach in general include later trying to integrate that script with other code that needs the default behavior (though CL support for :before/:after/:around auxiliary methods can help here, e.g. if I can't just subclass I could insert a seam with an :around method branching between my custom implementation over the library's without having to overwrite the library's; and in the long-term the SICL implementation will show the way to first-class environments that can allow multiple versions of stuff to co-exist nicely). Or the library could update and change the protocol, breaking my hack when I update to it. Or in other situations there may be more complex changes, like if you modify a macro but want the new modifications to apply to code using the old def, you need to redefine that old code, or if you redefine a class and want/need to specially migrate pre-existing instances you need to write an "update-instance-for-redefined-class" specializer, or if the changes just span across a lot of areas it may be infeasible to cherry-pick them into a 'patch' file/section of your script, so you're faced with choices on how much you want to fork the files of the lib and copy them into your own project to load. But anyway, all those possible problems are on me.

The asdf noise isn't that big of a deal and I think is only a little related here technically since it's a rather special library situation. It's more 'interesting' socially and as a show of possible conflict from having a core piece of infrastructure provided by the implementation, but not owned/maintained by the implementation. An analogous situation would arise if gcc, clang, and visual studio all agreed to use and ship versions of musl for their libc with any other libcs being obsolete and long forgotten. A less analogous situation is the existing one that Linux distributions do -- sometimes they just distribute, sometimes they distribute-and-modify, whether they are the first place to go to report issues or whether they punt it off to upstream depends case-by-case.