Interestingly, common lisp does allow one to write unsound programs intentionally. The `the` special operator (http://clhs.lisp.se/Body/s_the.htm#the http://clh

Interestingly, common lisp does allow one to write unsound programs intentionally. The `the` special operator (http://clhs.lisp.se/Body/s_the.htm#the http://clhs.lisp.se/Body/s_the.htm#the) declares that the form in the second position is of the type specified by the first position. You can imagine that this allows the runtime to crash early if the type is wrong, and you'd be right... But depending on the compiler you use, it also allows for the creation of unsound programs because you can set your flags such that the compiler uses the presence of `the` to make assumptions on the underlying types, letting the program evaluate faster by throwing out some runtime type checks.