Comment by shiro

Quoting the author:

    With Lisp it's always been non-trivial to know when a 
    function is tail-recursive. You can't just blindly count 
    parens, you have to try to "run" the function in your mind.

To me, Lisp code is just like a tree (well, sometimes DG) and it's statically obvious where the leaves are---no need to "run" the function in mind. But maybe it's just because I get used to it; once you learned, you forget what you saw (or you didn't see) before.