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You can cause the debugger to be called at a certain point in your
program by writing the expression (debug) at that point. To do
this, visit the source file, insert the text ‘(debug)’ at the
proper place, and type C-M-x (eval-defun, a Lisp mode key
binding). Warning: if you do this for temporary debugging
purposes, be sure to undo this insertion before you save the file!
The place where you insert ‘(debug)’ must be a place where an
additional form can be evaluated and its value ignored. (If the value
of (debug) isn’t ignored, it will alter the execution of the
program!) The most common suitable places are inside a progn or
an implicit progn (see Sequencing).
If you don’t know exactly where in the source code you want to put
the debug statement, but you want to display a backtrace when a
certain message is displayed, you can set debug-on-message to a
regular expression matching the desired message.