More cars

It's time to add more cars:

moreCars := LoadMoreCars()

PrintCars("More Cars, Domestic, Numeric, GM",
       cars.AddCars(moreCars).
              Filter(ByDomestic()).
              Filter(ByHasNumber()).
              Filter(ByMake("GM")))

The output of this is as follows:

More Cars, Domestic, Numeric, GM
-----------------------
car: GM Hummer H2
car: GM Hummer H3
car: GM Oldsmobile Delta 88
car: GM Oldsmobile 442

Wait, what? AddCars? How is that an HOF? AddCars neither takes a function nor returns a function. Even worse, it mutates the cars collection.