 In his article The Case for Case, Charles Fillmore studied the relationship between grammatical functions such as subject or object and deep case, today known as thematic relations, such as agent or theme. According to Fillmore, each verb selects a certain number of deep cases. For instance, the verb open in English requires an agent, a goal and an instrument as in the janitor opened the door with a key. Deep cases are subject to certain constraints such as that they can occur only once per sentence. Additionally, cases can be obligatory or optional. Obligatory cases may not be deleted at the risk of producing ungrammatical sentences. For example, the janitor opened is ungrammatical in this sense. Furthermore, Fillmore suggested a hierarchy for a universal subject selection rule which means, if the case frame of a verb contains an agent, this agent is realized as a subject of an active sentence. Otherwise, the deep case following the agent in the hierarchy, here the instrumental is promoted to subject. Fillmore's approach towards case has significantly influenced contemporary linguistics to the extent that numerous linguistic theories incorporate deep case roles in one or other form, such as the thematic structure in Chomsky's government and binding theory. It has also inspired the development of frame-based representations in artificial intelligence research.