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
(debug) isn't ignored, it will alter the execution of the
program!) The most common suitable places are inside a
progn (see Sequencing).