Like three different kinds, too!

        // Old-style generics; monomorphized 
        // with S as an "input" type of MyTrait
        fn foo<S, T: MyTrait<S>>(elem: T) { ... }

        // Where-clause-style generics; monomorphized, 
        // with S as an "output" (associated) type of MyTrait
        fn foo<S, T>(elem: T) 
        where T: MyTrait<Thing = S> { ... }

        // Trait objects; dynamic dispatch
        fn foo(elem: Box<MyTrait>) { ... }