R4RS 6.3  (caar pair)    ==>  object
          (cadr pair)    ==>  object
          ...
          (cdddar pair)  ==>  object
          (cddddr pair)  ==>  object

These procedures are compositions of car and cdr, where for example
caddr could be defined by

(define (caddr x) (car (cdr (cdr x)))).

Arbitrary compositions, up to four deep, are provided. There are
twenty-eight of these procedures in all.
