So far when I have asked you to type in a list of numbers I have been assuming that you will type the list correctly. If you made an error you had to retype the entire def expression. However it would be nice to be able to replace the values in a list after you have typed it in. Suppose you would like to change the 12 in the list x used in the Section 4.3 to 11. A note of caution is needed here.
Lisp symbols are merely labels for different items.
When you assign a name to an item with the def command you are not producing a new item. Now x and y refer to different items and changes to x will not affect y .