  Much more complex logical issues arise when the organization of a domain into hierarchies is allowed to have exceptions. One way to approach this topic is to explore how to make a taxonomic logic nonmonotonic; but nonmonotonic inheritance is a topic in its own right. Although there are strong affinities to nonmonotonic logic, nonmonotonic logic relies more heavily on graph-based representations than on traditional logical ideas, and seems to provide a much finer-grained approach to nonmonotonic reasoning that raises entirely new issues, and which quickly becomes problematic. For this reason, systems of nonmonotonic inheritance tend to be expressively weak, and their relations to the more powerful nonmonotonic logic has never been fully clarified. For background on this topic, see  Thomason 1992,  Horty 1994.