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Numbers, Lists inside of Lists

Lists can also have numbers in them, as in this list: (+ 2 2). This list has a plus-sign, ‘+’, followed by two ‘2’s, each separated by whitespace.

In Lisp, both data and programs are represented the same way; that is, they are both lists of words, numbers, or other lists, separated by whitespace and surrounded by parentheses. (Since a program looks like data, one program may easily serve as data for another; this is a very powerful feature of Lisp.) (Incidentally, these two parenthetical remarks are not Lisp lists, because they contain ‘;’ and ‘.’ as punctuation marks.)

Here is another list, this time with a list inside of it:

     '(this list has (a list inside of it))

The components of this list are the words ‘this’, ‘list’, ‘has’, and the list ‘(a list inside of it)’. The interior list is made up of the words ‘a’, ‘list’, ‘inside’, ‘of’, ‘it’.