It is not clear that the nightmare outlined above applies to actual programming. I am a little weak on my Lisp-1, but IIUC the above changes < to be whatever comparison operator got passed to sort, so the author of the above sort should be fired for breaking <. As Arc matures it might not hurt for the compiler to issue a warning over such sabotage, which is really what it is so I am not sure why we are discussing it as a Real World Problem.
As for "sequence" and someone depending on some /other/ higher level binding, ie, it is capturing the variable sequence by design, (a) intentional variable capture is as rare as it is powerful and the premise is that one has a suite of cooperating macros that work together to bind and then use the rare variable designated for capture, so one is not invoking one of these macros in isolation unaware of how it works or what it captures. A good example from my code at one time was the binding and capture of SELF, being used in some OO wizardry to stand for the "instance at hand" a la smalltalk. If I was rebinding self outside the cooperating suite of macros (which I did, rarely) it was because I had coded myself into a corner, but I knew damn well what I was doing and why I was doing it and I still crossed my fingers.
btw, a trick the CL crowd uses for lexical vs dynamic binding (the latter being an opportunity for a similar kind of confusion as the one you fear) is simply to name dynamic variables +like-this+. (not really, we use asterisks, but this forum would just show like-this if I bracket it with asterisks.) That simple convention solves the whole problem, as long as we fire anyone naming a scratch variable that way. :)
