def _txx_P(P, xy):
    """Define the boundary between 3x-3y, T=f(P)

    Parameters
    ----------
    P : float
        Pressure, [MPa]
    xy: string
        Subregions options: cd, gh, ij, jk, mn, qu, rx, uv

    Returns
    -------
    T : float
        Temperature, [K]

    References
    ----------
    IAPWS, Revised Supplementary Release on Backward Equations for Specific
    Volume as a Function of Pressure and Temperature v(p,T) for Region 3 of the
    IAPWS Industrial Formulation 1997 for the Thermodynamic Properties of Water
    and Steam, http://www.iapws.org/relguide/Supp-VPT3-2016.pdf, Eq. 1

    Examples
    --------
    >>> _txx_P(25,"cd")
    649.3659208
    >>> _txx_P(23,"gh")
    649.8873759
    >>> _txx_P(23,"ij")
    651.5778091
    >>> _txx_P(23,"jk")
    655.8338344
    >>> _txx_P(22.8,"mn")
    649.6054133
    >>> _txx_P(22,"qu")
    645.6355027
    >>> _txx_P(22,"rx")
    648.2622754
    >>> _txx_P(22.3,"uv")
    647.7996121
    """
    ng = {
        "cd": [0.585276966696349e3, 0.278233532206915e1, -0.127283549295878e-1,
               0.159090746562729e-3],
        "gh": [-0.249284240900418e5, 0.428143584791546e4, -0.269029173140130e3,
               0.751608051114157e1, -0.787105249910383e-1],
        "ij": [0.584814781649163e3, -0.616179320924617, 0.260763050899562,
               -0.587071076864459e-2, 0.515308185433082e-4],
        "jk": [0.617229772068439e3, -0.770600270141675e1, 0.697072596851896,
               -0.157391839848015e-1, 0.137897492684194e-3],
        "mn": [0.535339483742384e3, 0.761978122720128e1, -0.158365725441648,
               0.192871054508108e-2],
        "qu": [0.565603648239126e3, 0.529062258221222e1, -0.102020639611016,
               0.122240301070145e-2],
        "rx": [0.584561202520006e3, -0.102961025163669e1, 0.243293362700452,
               -0.294905044740799e-2],
        "uv": [0.528199646263062e3, 0.890579602135307e1, -0.222814134903755,
               0.286791682263697e-2]}

    n = ng[xy]
    Pr = P/1
    T = 0
    for i, ni in enumerate(n):
        T += ni * Pr**i
    return T
Define the boundary between 3x-3y, T=f(P)

    Parameters
    ----------
    P : float
        Pressure, [MPa]
    xy: string
        Subregions options: cd, gh, ij, jk, mn, qu, rx, uv

    Returns
    -------
    T : float
        Temperature, [K]

    References
    ----------
    IAPWS, Revised Supplementary Release on Backward Equations for Specific
    Volume as a Function of Pressure and Temperature v(p,T) for Region 3 of the
    IAPWS Industrial Formulation 1997 for the Thermodynamic Properties of Water
    and Steam, http://www.iapws.org/relguide/Supp-VPT3-2016.pdf, Eq. 1

    Examples
    --------
    >>> _txx_P(25,"cd")
    649.3659208
    >>> _txx_P(23,"gh")
    649.8873759
    >>> _txx_P(23,"ij")
    651.5778091
    >>> _txx_P(23,"jk")
    655.8338344
    >>> _txx_P(22.8,"mn")
    649.6054133
    >>> _txx_P(22,"qu")
    645.6355027
    >>> _txx_P(22,"rx")
    648.2622754
    >>> _txx_P(22.3,"uv")
    647.7996121