class X:
    pass


class Y(X):
    pass


class Z(X, Y):
    pass


xyz = Z()

# L[X] = [X] + merge([X])
# L[X] = [X]

# L[Y] = [Y] + merge(L(X), [Y])
# L[Y] = [Y] + merge([X], [Y])
# L[Y] = [Y, X] + merge([Y])
# L[Y] = [Y, X]

# L[Z] = [Z] + merge(L(X), L(Y), [X, Y])
# L[Z] = [Z] + merge([X], [Y, X], [X, Y])
# >> cannot pick X
# >> cannot pick Y