The original definition by McCarthy was syntactical rather than semantical. Nevertheless, this important dichotomy is semantical, rather than expressed by purely grammatical means. In other classes, semantical regularities are mostly absent or are obscured by many exceptions. In many instances a prefixed verb has no apparent semantical relationship with the primary verb. Again, in collaboration with Imielinski, Lipski studied the semantical issues of relational databases. The episodic buffer is also assumed to have links to long-term memory and semantical meaning. There are ways of using non-normal worlds in a semantical system that invalidate contraction. Possible worlds approach enabled more exact study of semantical questions.