def _unwhiten_cf(cf, x):
    """
    Unwhiten a matrix

    Parameters
    ----------
    cf : array_like(ndim=2, dtype=float)
        The upper triangular part of the cholesky decomposition of a

    x : array_like(ndim=2, dtype=float)
        Not sure, just blindly porting...

    Returns
    -------
    updated matrix.

    """
    return cf.T.dot(x)
