How do I assert activity counts in a workflow trace after a unit test?

I am unit testing a WF4 service using the Unit testing Nuget package. There is a functionality for asserting that stuff does or does not exist in the end-of-test tracking information.

Consider the following unit test code:

WorkflowServiceTestHost host = null;
using (host = CreateHost())
{
    var proxy = new ServiceClient(Binding, ServiceAddress);
// Call receive points in the workflow

}
finally
{
    host.Tracking.Trace();
    // I would like to assert stuff like the WF did not abort
    // For now, I would just like to assert that there are a 
    // certain number of invocations of a specific activity.
    // I can find no examples of how to call this:
    host.Tracking.Assert.ExistsCount(?????what goes here???);
}

How does one call ExistsCount()? The prototype looks like this:

public void ExistsCount<TRecord>(Predicate<TRecord> predicate, int count, string message = null) where TRecord : TrackingRecord

but I can find no examples or documentation to understand what is expected for the predicate.