I believe it stems from an early idea that not specifying it might allow for the compiler to do some optimisations (same reason it is not specified in C). I thi

I believe it stems from an early idea that not specifying it might allow for the compiler to do some optimisations (same reason it is not specified in C). I think it also allows for some freedom in the internal representation for continuation rewriting, and that it now would mean some problems for implementations if a certain way was specified. (Please correct me if I'm wrong about that). It only really matters if you are passing arguments to a function that both read and mutate the same variable. In idiomatic scheme that really only happens with IO or something like srfi-121 styled generators.