def D_to_nu(D):
    r"""True anomaly from parabolic anomaly.

    Parameters
    ----------
    D : float
        Eccentric anomaly.

    Returns
    -------
    nu : float
        True anomaly.

    Notes
    -----
    From [1]_:

    .. math::

        \nu = 2 \arctan{D}

    """

    return 2.0 * np.arctan(D)