class Atom:
    car: Atom | int | str | bool | None
    cdr: Atom | None

    def __init__(self, car: Atom|int|str|bool|None, cdr: Atom|None):
        self.car = car
        self.cdr = cdr

    def setval(self, atom: Atom | int | str | bool | None):
        self.car = atom

    def append(self, atom: Atom):
        self.cdr = atom

    def __str__(self):
        if type(self.car) is Atom:
            return "(" + self.car.__str__() + ("" if self.cdr is None else (" " + self.cdr.__str__())) + ")"
        elif self.car is None:
            return "()"
        else:
            return self.car + ("" if self.cdr is None else self.cdr.__str__())