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The most unusual features of the SBCL compiler (which is very similar to the original CMUCL compiler, also known as Python) is its unusually sophisticated understanding of the Common Lisp type system and its unusually conservative approach to the implementation of type declarations.
These two features reward the use of type declarations throughout development, even when high performance is not a concern. Also, as discussed in the chapter on performance (see Efficiency), the use of appropriate type declarations can be very important for performance as well.
The SBCL compiler also has a greater knowledge of the Common Lisp
type system than other compilers. Support is incomplete only for types
involving the satisfies type specifier.