If we want to modify a list and also keep a copy of the original, we need to be able to make a copy of the list itself, not just the reference. This process is sometimes called cloning, to avoid the ambiguity of the word copy.
The easiest way to clone a list is to use the slice operator.
Taking any slice of a creates a new list. In this case the slice happens to consist of the whole list.
Now we are free to make changes to b without worrying about a. Again, we can clearly see in codelens that a and b are entirely different list objects.