a high-level multi-platform, thread-safe C/C++ windowing and graphics library built upon a fast, low-level graphics kernel. (new feature is FGX - dynamic GUI via XML file aka XUI file format)
Guy Steele's keynote at the 1998 ACM OOPSLA conference on "Growing a Language" (mostly about JAVA) discusses the importance of and issues associated with designing a programming language that can be grown by its users.