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