There are libraries that add both of these features at once (either by defining new (meta)metaclasses or by simulating same effect through simple copying of met

There are libraries that add both of these features at once (either by defining new (meta)metaclasses or by simulating same effect through simple copying of methods from base classes.) On the other hand, Smalltalkers are quite keen on adding new methods to almost any existing system class, so mixins are not often as useful. Also Smalltalk development often relies on IDEs that do pretty extensive code generation behind the scenes and I think that this image based development with no real source code to speak of, and lots of reflection based development tools, is more important aspect of Smalltalk than any OO features.