I. Introduction
Consider the scenario where a computer historian is interested in preparing an article on university professors who have designed a programming language and also won an award in Computer Science. If the historian only knows of a few professor-university-award triples, to start writing the article she must have a more comprehensive list of such triples. In general, users are interested in finding entities of various types (e.g., persons, products, organizations) that are related in certain ways. Tasks like the above one are becoming increasingly common in several applications, including search, recommendation systems, and business intelligence.