* Implement class Foo;
* Implemet subclass { ... }, meaning class Foo extends OuterClass { ... } for
  nested classes.
* Allow $class = extends Foo { ... }; and $class = with Role { ... }; for
  creating anonymous classes. Not only $class = class extends Foo { .. };
  doesn't read very well, it's also hard to distinguish from creating a class
  called 'extends'