def simple_sphere(xy):
return xy<EOL>
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