[reikonomusha]

This is dynamic scope, not lexical. They are not equivalent. You might further attempt to solve this by locally binding a lexical variable of the same name to the special variable, in order to “recreate” the lexical environment. (A quicker way to do this would be to pass these values to the branch functions directly.)

You will quickly find that this also will not work, for if a closure was made inside of each branch, the environment would not be shared among them.

The original intent of this exercise was to demonstrate you can’t macro your way out of all deficiencies in Lisp. And certainly in this case, even if we were to accept this dynamic variable rigamarole, we would find that we could not write a macro that, e.g., replaces CASE.