package system and backwards compatibility

I'm not sure it can be solved immediately, but I'd might as well make an note of it for those who ponder on the packaging system. I just submitted a patch for a new builtin function in XEmacs, partially in order to be able to completely sync a lisp file in a package with Emacs21. Now, should the code in the package assume this new function is available? Should it detect lack of the function, and add a workaround, for it? If packages aren't allowed to use new functionality in XEmacs, the bit-rot of xemacs-packages will only increase and maintainance will be a nightmare. IMHO this must be avoided, packages must be allowed to assume recent features in XEmacs. If packages are allowed to use new functionality in XEmacs, newer packages won't work in older XEmacses. I see one reasonable solution to this: Stuff code for older releases into a library, and make sure this library is updated if any other packages is downloaded. Maybe this is what APEL does? If so, I think the packaging system must make sure that recent APEL version is installed if other packages are upgraded. If the current package system already handle this situation, please point it out because I haven't seen it.