Roto objects for obscuring tracks

How do you deal with tracking a spline object that will be partially obscured at some point. In Mocha, the higher splines in the list are ignored by those under it. It doesn’t seem to work that way in Silhouette Paint. I have a spline that is going to go behind an object. If I made a second spline for the foreground object, how can I get the background spline to ignore the foreground object?

See the Exclude Areas While Planar Tracking section here:

Thank you. That is exactly what I was looking for.