A good implementation also has total dynamic type checking, and the better allow you to relax that for run time speed (although it was nearly free on Lisp Machi

A good implementation also has total dynamic type checking, and the better allow you to relax that for run time speed (although it was nearly free on Lisp Machines and can be pretty cheap on out of order superscalar CPUs like Intels, as long as you don't enable Hyperthreading which tends to use up the otherwise free execution engines that get used for it).