When the debugger is entered, it displays the previously
selected buffer in one window and a buffer named ‘*Backtrace*’
in another window. The backtrace buffer contains one line for
each level of Lisp function execution currently going on. At the
beginning of this buffer is a message describing the reason that
the debugger was invoked (such as the error message and
associated data, if it was invoked due to an error). The
backtrace buffer is read-only and uses a special major mode,
Debugger mode, in which letters are defined as debugger
commands. The usual Emacs editing commands are available; thus,
you can switch windows to examine the buffer that was being
edited at the time of the error, switch buffers, visit files, or
do any other sort of editing. However, the debugger is a
recursive editing level (see Recursive Editing) and it is wise
to go back to the backtrace buffer and exit the debugger (with
the q command) when you are finished with it. Exiting the
debugger gets out of the recursive edit and kills th