def simple_sphere(xy):
    """
    A simple sphere, as a set of residuals (cost = sum(residuals**2))

    Simply returns the input parameters. A linear, uncoupled model.

    Parameters
    ----------
        - xy : 2-element list-like
            The x,y parameters of the model.

    Returns
    -------
        - xy : 2-element list-like
            The residuals, which are just xy
    """
    return xy