Comment 4 for bug 1026439

BTW, here's some discussions about whether the result of backquote is considered mutable or not. The spec is not very clear, but it seems like the consensus is that backquote can return literal data whenever it feels like it. Which is good, because SBCL already does that in many cases. This just adds a few more cases. :)

https://groups.google.com/group/comp.lang.lisp/browse_thread/thread/74c41c41aeb80528
https://groups.google.com/group/comp.lang.lisp/browse_thread/thread/43e14ad1ba72dd0d
https://groups.google.com/group/comp.lang.lisp/browse_thread/thread/385f2adf7a3175fd