Notes on computational linguistics.pdf - UCLA Department of ...

Stabler - Lx 185/209 2003
9 Beyond context free: a first small step
(1) Many aspects of language structure seem to be slightly too complex to be captured with context free
grammars. Many different, more powerful grammar formalisms have been proposed, but almost all of
them lie in the class that Joshi and others have called “mildly context sensitive” (Joshi, Vijay-Shanker, and
Weir, 1991; Vijay-Shanker, Weir, and Joshi, 1987).
This class includes certain Tree Adjoining Grammars (TAGs), certain Combinatory Categorial Grammars
(CCGs), and also a certain kind of grammar with movements (MGs) that that has been developed since
1996 by Cornell, Michaelis, Stabler, Harkema, and others.
It also includes a number of other approaches that have not gotten so much attention from linguists:
Linear Context Free Rewrite Systems (Weir, 1988), Multiple Context Free Grammars (Seki et al., 1991),
Simple Range Concatenation Grammars (Boullier, 1998). As pointed out by Michaelis, Mönnich, and
Morawietz (2000), these grammars are closely related to literal movement grammars (Groenink, 1997),
local scattered context languages (Greibach and Hopcroft, 1969; Abramson and Dahl, 1989), string generating
hyperedge replacement grammars (Engelfriet, 1997), deterministic tree-walking tree-to-string
transducers (Kolb, Mönnich, and Morawietz, 1999), yields of images of regular tree languages under
finite-copying top-down tree transductions, and more! The convergence of so many formal approaches
on this neighborhood might make one optimistic about what could be found here.
These methods can all be regarded as descending from Pollard’s (1984) insight that the expressive
power of context-free-like grammars can be enhanced by marking one or more positions in a string
where further category expansions can take place. The tree structures in TAGs and in transformational
grammars play this role.
This chapter will define MGs, which were inspired by the early “minimalist” work of Chomsky (1995),
Collins (1997), and many others in the transformational tradition. There are various other formal approaches
to parsing transformational grammar, but this one is the simplest. 40
40 See, for example, Marcus (1980), Berwick and Weinberg (1984), Merlo (1995), Crocker (1997), Fong (1999), Yang (1999). Rogers
and Kracht have developed elegant formal representations of parts of transformational grammar, but no processing model for these
representations has been presented (Rogers, 1995; Rogers, 1999; Kracht, 1993; Kracht, 1995; Kracht, 1998).
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