Problem: the internal of debug

[Shenli Zhu]

Hi all,

I want to know the internal of debug. So I add just one (debug) in the entrance of test function, and step into it. 

What I don't understand is, after I press a key (e.g. step), the step 
function will call (exit-recursive-edit), and debug function kill 
backtrace and exit. How can it receive more keys and step into test function (remember I just add one (debug) in the entrance of a test function)?

debug can be summarized to steps:
1. save parameter
2. generate *backtrace* buffer, change it to debugger-mode, 
3. receive key stroke and do step/cont/etc
4. kill *backtrace*, restore parameter

The lines of code about step 3 seems to be
======
    (let ((standard-output nil)
              (buffer-read-only t))
          (message "")
          ;; Make sure we unbind buffer-read-only in the right buffer.
          (save-excursion
            (recursive-edit)))
======

Thanks,
Shenli