In SBCL, type declarations are treated as assertions. If the compiler is sure the assertion is valid without doing runtime checks, it can remove them. The optim

In SBCL, type declarations are treated as assertions. If the compiler is sure the assertion is valid without doing runtime checks, it can remove them. The optimization levels are also taken into account (http://www.sbcl.org/manual/index.html#Handling-of-Types http://www.sbcl.org/manual/index.html#Handling-of-Types).