Yes. With macros and runtime logic it's possible to implement just about any type system in Lisp, even an ML-like system. Box everything and use macros to transparently pick it apart and reconstruct your wrapped type objects when you use them. My initial reaction to that whole idea was horror. The inefficiency! But maybe not necessarily. A compiler that understood the type system used, could optimize most of it away. Just like how Haskell or SML compilers do.