“…In many applications such as mathematics and natural language processing, however, the instances are naturally represented as expressions or parse trees. For example, in symbolic integration, one might want to learn the set of expressions for which integration by parts is a good approach to use [Mitchell, Utgoff, & Banerji, 19831. In natural language processing, one might want to identify all (parsed) messages which contain a reference to a job opportunity or an automobile accident [Cardie, 19971. In many cases, such sets of expressions can be naturally represented as a union of "tree patterns".…”