struct SEXPR {    struct SEXPR * sexpr_car;    struct SEXPR * sexpr_cdr;};typedef struct SEXPR * sexpr;extern sexpr cons();# define nil ((sexpr) 0)# define car(x) ((x) -> sexpr_car)# define cdr(x) ((x) -> sexpr_cdr)# define null(x) ((x) == nil)# define head(x) car(x)# define tail(x) cdr(x)# define caar(x) car(car(x))# define cadr(x) car(cdr(x))# define cddr(x) cdr(cdr(x))# define cdar(x) cdr(car(x))# define caddr(x) car(cdr(cdr(x)))# define first(x) car(x)# define second(x) cadr(x)# define third(x) caddr(x)# define list1(x) cons(x, nil)# define list2(x,y) cons(x, cons(y, nil))# define list3(x,y,z) cons(x, cons(y, cons(z, nil)))