 The authors are very grateful to the two SEP referees, Tom Wasow and William Starr, who provided careful reviews of our drafts; to Bonnie Webber and Zoltan Galsi for insightful comments and advice; and to Dean Mellow for some helpful corrections. BCS was the lead author of this article throughout the lengthy period of its preparation, and worked on it in collaboration with FJP and GKP until the post-refereeing revision was submitted at the end of April 2011. She died two weeks later, on May 14. FJP and GKP oversaw the few final corrections that were made when the HTML version was produced in September 2011.