Class extension - side effects

Added on behalf of @bulajam

Using extend on a class implies two passes (not in the sense of file access) over all involved source files. First pass will effectively collect all attributes which anyone added to the class. Second pass can start instantiating objects, otherwise all instances of the class will not have same members. This would need to be mentioned in the manual.

Also "id" of class car here would be somehow different to "id" of the extended car class (if it ever comes to e.g. serializing/deserializing of runtime objects).