This Common Lisp code represents a recursive
factorial function. The program computes 4! when
run, though the argument "4" can be switched out
with any other number without a problem (good
luck finding out where to substitute it...)

Run the code with clisp, either by giving the file
as a command line argument, $ clisp file.lisp
or executing the following command within a clisp session.

$ (load "file.lisp")
where "file.lisp" is the source file name.