Common Lisp/Beyond ANSI Common Lisp

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The implementation-dependent features which exist on most implementations but not a part of ANSI Common Lisp standard.

1. OS - communicating with the underlying OS (shell access, etc.)
2. Streams - user-defined streams (a.k.a. Gray Streams)
3. Sockets - using sockets in various implementations
4. MOP - Meta Object Protocol
5. FFI - Foreign Function interface (using CFFI compatibility layer)
6. Threads - writing multi-threaded programs