Editing Lisp (programming language) (section)

====S-expressions represent lists====
Parenthesized S-expressions represent linked list structures. There are several ways to represent the same list as an S-expression. A cons can be written in ''dotted-pair notation'' as {{Lisp2|(a . b)}}, where {{Lisp2|a}} is the car and {{Lisp2|b}} the cdr. A longer proper list might be written {{Lisp2|(a . (b . (c . (d . nil))))}} in dotted-pair notation. This is conventionally abbreviated as {{Lisp2|(a b c d)}} in ''list notation''. An improper list<ref name="r3sL3">NB: a so-called "dotted list" is only one kind of "improper list". The other kind is the "circular list" where the cons cells form a loop. Typically this is represented using #n=(...) to represent the target cons cell that will have multiple references, and #n# is used to refer to this cons. For instance, (#1=(a b) . #1#) would normally be printed as ((a b) a b) (without circular structure printing enabled), but makes the reuse of the cons cell clear. #1=(a . #1#) cannot normally be printed as it is circular, although (a...) is sometimes displayed, the CDR of the cons cell defined by #1= is itself.</ref> may be written in a combination of the two – as {{Lisp2|(a b c . d)}} for the list of three conses whose last cdr is {{Lisp2|d}} (i.e., the list {{Lisp2|(a . (b . (c . d)))}} in fully specified form).