"Entirely" is hyperbolic here. Building new numeric types with CL is certainly possible if you write your own versions of numeric functions like `plus` in lieu of the builtin `+`, and it's easy to make your new functions backward-compatible so `plus` will work on e.g. integers. This is not as easy as if the numeric functions were generic but it's still doable. By not being generic the builtin CL numeric functions run very fast on the most common number types; that was the tradeoff benefit of not designing them be generic.