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Abstract

Multiple context-free grammars (mcfg's) and head grammars (hg's) are subclasses of generalized context-free grammars introduced by Pollard to describe the syntax of natural languages. In this paper, the following properties of mcfg's and hg's are shown: (1) the class HL of languages generated by hg's is the same as that generated by the tree adjoining grammars introduced by Joshi et al.; (2) HL is the same as that generated by left-wrapping hg's (or right-wrapping hg's) which is a proper subclass of hg's; (3) HL is properly included in the one generated by 2-mcfg's, which is a proper subclass of mcfg's. As a corollary of (1), it is also shown that HL is a substitution closed full AFL.