Overlord programs

Suppose that i have an overlord program that has a function who
manipulates lists . After a while the program starts counting to -let's
say- 10,000). When it reaches 10,000 it decides that it might be
interesting to do a little arithmetic and wants to create a function
that adds 3 numbers. How does the creation of this function is going to
be implemented by the program ? Possibly the program will write down a
body for the function and then is going to bind it with a funtion name.
How is this accomplisehd ? I wandered the HyperSpec for about 4 days
and i could't find a clear way to do it. i am getting kind of lost in
all these parameters and names. I heard that this is one of the most
fundmanental benefits of using Lisp so it shouldn't be hidden under all
these technicalities but instead it should be right at the fron of the
manual!Can you please help me? Thank you

.