The documents available from my homepage are provided as a means to ensure timely dissemination of scholarly and technical work on a non-commercial basis. Copyright and all rights therein are maintained by the authors or by other copyright holders, notwithstanding that they have offered their works here electronically. It is understood that all persons copying this information will adhere to the terms and constraints invoked by each author's copyright. These works may not be reposted without the explicit permission of the copyright holder. Also, some of these works have been submitted for publication. Copyright may be transferred without further notice and this version may no longer be accessible. In some cases, the postscript file provided here is of a slightly earlier version of the paper than the published one; the differences, if any, are minor. In particular, copyright of some of these works (specifically those appearing in ACM conference proceedings and journals) is held by the Association for Computing Machinery, Inc. Permission to make digital or hard copies of part or all of these works for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that new copies bear this notice and the full citation on the first page. Copyrights for components of these works owned by others than ACM must be honored. Abstracting with credit is permitted. To copy otherwise, to republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. Request Permissions from Publications Dept, ACM Inc., Fax +1 (212) 869-0481, or permissions@acm.org. Also, copyright of some other of these works (specifically those appearing in LNCS series http://www.springer.de/comp/lncs/index.html and other conference proceedings and journals) is held by Springer-Verlag, and they should be understood to carry the "© Springer-Verlag" notice.