In Lisp it is relatively easy to change a function. A typical thing is 'advising', where some code is added to a function. This had been done to ordinary functi

In Lisp it is relatively easy to change a function. A typical thing is 'advising', where some code is added to a function. This had been done to ordinary functions decades ago and Common Lisp added it as a standard to its object system. > Modifying code is a step up. For example, in lisp, it is fairly easy to grab a function, walk its tree for use of integers, and replace every '3' by '4' and every usage of the '+' function by '-' Actually it is not easy. Actually walking Lisp code is relatively complex. The code walker needs to understand built-in Lisp constructs, macros and more.