class A:
    def __init__(self, a):
        self._a = a

class B:
    def __init__(self, b):
        self._b = b

class C:
    def __init__(self, c):
        self._c = c

def hovedprogram():
    c = A("B")
    a = B("C")
    b = C("A")
    b = a
    a = c
    c = b

hovedprogram()