1/6, 2: 1/6, 3: 1/6, 4: 1/6, 5: 1/6, 6: 1/6}
    >>> density(2*D).dict
    {2: 1/6, 4: 1/6, 6: 1/6, 8: 1/6, 10: 1/6, 12: 1/6}
    >>> density(X)(x)
    sqrt(2)*exp(-x**2/2)/(2*sqrt(pi))
    r