NATURAL EXPERT is a product that allows users to build knowledge-based systems. It uses a lazy functional language, NATURAL EXPERT LANGUAGE, to implement backward chaining and provide a reliable knowledge processing environment in which development can take place. Customers from all over the world buy the system and have used it to handle a variety of problems, including applications such as airplane servicing and bank loan assessment. Some of these are used 10,000 times or more per month.