Both the compiler and the interpreter are ``properly tail recursive.'' If a function call is in a tail-recursive position, the stack frame will be deallocated at the time of the call, rather than after the call returns. Consider this backtrace:
(defun foo2 (...) ... (bar ...))
(defun bar (...) ...)
Usually the elimination of tail-recursive frames makes debugging more pleasant, since theses frames are mostly uninformative. If there is any doubt about how one function called another, it can usually be eliminated by finding the source location in the calling frame (section 3.5.)
For a more thorough discussion of tail recursion, see section tail-recursion.