A recursive program calls itself as a subroutine. Recursion allows one to write programs that are powerful, yet simple and elegant. Often, a large problem can be handled by a small program which:
(defun factorial (n) (if (<= n 0) 1 (* n (factorial (- n 1))) ) )
Rule: Make sure that each recursive call involves an argument that is strictly smaller than the original; otherwise, the program can get into an infinite loop.
A good method is to use a counter or data whose size decreases with each call, and to stop at 0; this is an example of a well-founded ordering.