Some forms of nonatomic S-expressions arise so frequently there is a special notation for them. A dotted-pair of the form (
*S*
_{1} . (
*S*
_{2} . (· · · . (
*S*
_{k} . NIL)· · ·))) where *S*
_{1}, *S*
_{2}, . . . , *S*
_{k} are all S-expressions is called a *list* and is written as (
*S*
_{1} *S*
_{2} ... *S*
_{k}
), which is the sequence of S-expressions
*S*
_{1}, *S*
_{2}, . . . , *S*
_{k} written with intervening blanks and enclosed in parentheses. There is no confusion with dot notation since there are no dots between the *S*
_{i} elements in the list. There may, of course, be dots within some or all of the elements *S*
_{1}, . . . , *S*