I believe it is legal to actually reassign the global builtins, not just shadow them. Proving that a program doesn't do this is difficult to impossible, and I t

I believe it is legal to actually reassign the global builtins, not just shadow them. Proving that a program doesn't do this is difficult to impossible, and I think some schemes give you an option to tell the compiler you won't do it. Chez Scheme has a section in their manual recommending pulling top level declarations into lets that it can analyze locally in order to optimize them.