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(emacs-lisp-intro.info)debug-on-quit

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`debug-on-quit' and `(debug)'
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   In addition to setting `debug-on-error' or calling `debug-on-entry',
there are two other ways to start `debug'.

   You can start `debug' whenever you type `C-g' (`keyboard-quit') by
setting the variable `debug-on-quit' to `t'.  This is useful for
debugging infinite loops.

   Or, you can insert a line that says `(debug)' into your code where
you want the debugger to start, like this:

     (defun triangle-bugged (number)
       "Return sum of numbers 1 through NUMBER inclusive."
       (let ((total 0))
         (while (> number 0)
           (setq total (+ total number))
           (debug)                         ; Start debugger.
           (setq number (1= number)))      ; Error here.
         total))

   The `debug' function is described in detail in Note: The Lisp
Debugger.