. Over the years , perhaps the most popular kind of data structure has been a hierarchy of nodes connected by IS-A links.. IS-A is a term used to describe the existence of a generalization relation between a subconcept like “dog” and its superconcept “mammal.” i.e. ,”Dog IS-A mammal.” The most popular kind of inference has involved the inheritance of information from the top levels of the hierarchy downward along these IS-A links. Such an organization allows information to be shared among many nodes and thus leads to large-scale representational economies. However , as W. Woods and L. Schubert et al. discuss, IS-A also raises a number of thorny formal issues , such as whether to distinguish information about classes from information about instances of classes and how to handle exceptions , which are still the subject of intense research.