Now we move on to function-lambda-expression.  While programmers might occasionally find a reason to want to print out a function’s source code from within the running program, be warned that this function is not guaranteed to produce helpful results.  While the implementation is encouraged to produce the lambda expression for the function, at least on non-compiled forms, it is permitted to return nil for any input.  Consequently, you cannot rely on this function in portable code, and should only use it in non-essential contexts after verifying that it does something interesting on the platform it’s running on at the time.