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To see how information is passed to functions, let's look again at our old standby, the addition of two plus two. In Lisp, this is written as follows:

(+ 2 2)

If you evaluate this expression, the number 4 will appear in your echo area. What the Lisp interpreter does is add the numbers that follow the  + .

The numbers added by  + are called the arguments of the function  + . These numbers are the information that is given to or passed to the function.

The word `argument' comes from the way it is used in mathematics and does not refer to a disputation between two people; instead it refers to the information presented to the function, in this case, to the  + . In Lisp, the arguments to a function are the atoms or lists that follow the function. The values returned by the evaluation of these atoms or lists are passed to the function. Different functions require different numbers of arguments; some functions require none at all.(1)

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