R4RS 6.3  (car pair)  ==>  object

Returns the contents of the car field of PAIR. Note that it is an
error to take the car of the empty list.

(car '(a b c))      ==>  a
(car '((a) b c d))  ==>  (a)
(car '(1 . 2))      ==>  1
(car '())           ==>  error
