Shieber, Moore and Pereira were supported in this work by a contract with the Nippon Telephone Telegraph Corp. and by a gift from the Systems Development Foundation as part of a coordinated research effort with the Center for the Study of Language and Information, Stanford University; van Noord was supported by the European Community and the Nederlands Bureau voor Bibliotheekwezen en Informatieverzorgin through the Eurotra project. We would like to thank Mary Dalrymple and Louis des Tombe for their helpful discussions regarding this work.