GNU Emacs Lisp Reference Manual
18.1.4 Explicit Entry to the Debugger
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. 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 section 10.1 Sequencing).