CLOS is dynamic, meaning that not only the contents, but also the "structure" of its objects can be modified at runtime. CLOS supports changing class definitions on-the-fly (even when instances of the class in question already exist) as well as changing the class membership of a given instance through the codice_13 operator. CLOS also allows one to add, redefine and remove methods at runtime. The Circle-Ellipse Problem is readily solved in CLOS, and most OOP design patterns either disappear or are qualitatively simpler.