The basic function that constructs new list structure is the function cons.

If y is a list, then we can think of (cons x y ) as adding the new element x to the front of the list y.

(cons 'a '(b))      =  (a b)

(cons 'a '())       =  (a)

(cons '(a) '(b))    =  ((a) b)

(cons 'a 'b)        =  (a . b)

The following axioms always hold:

1. (car (cons x y )) = x

2. (cdr (cons x y )) = y

Lisp guarantees that every pair made by cons is brand-new and distinct from any other pair.

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