1,  3,  6],
           [ 4,  9, 15]])

    ``cumsum(b)[-1]`` may not be equal to ``sum(b)``

    >>> b = np.array([1, 2e-9, 3e-9] * 1000000)
    >>> b.cumsum()[-1]
    1000000.0050045159
    >>> b.sum()
    1000000.0050000029

    r