Multiset-Valued Linear Index Grammars: Imposing Dominance Constraints on   Derivations
  This paper defines multiset-valued linear index grammar and unordered vector grammar with dominance links. The former models certain uses of multiset-valued feature structures in unification-based formalisms, while the latter is motivated by word order variation and by ``quasi-trees'', a generalization of trees. The two formalisms are weakly equivalent, and an important subset is at most context-sensitive and polynomially parsable. 
Owen Rambow (Universite Paris 7)