In 1971 the decision was made to begin computer generation of the British National Bibliography directly from MARC data. The plan was immediately delayed when it became evident that the strategy worked for all fields except the subject index field. Austin was called upon to design a strategy that would clear this critical hurdle.
Main entry is necessary only when shelf location is the predominant consideration.
Those statements are then analyzed linguistically, searching for such variables as terms denoting action, the object of the action, time, location, etc.