def METHOD_NAME(self, base_ring, n):
    """
    The generating series for the species of subsets is
    `e^{2x}`.
    EXAMPLES::
        sage: S = species.SubsetSpecies()
        sage: [S.generating_series().coefficient(i) for i in range(5)]
        [1, 2, 2, 4/3, 2/3]
    """
    return base_ring(2)**n / base_ring(factorial(n))