COMMUNICATIONS OF THE ACM Trusted insights for computing’s leading professionals.

Communications of the ACM is the leading monthly print and online magazine for the computing and information technology fields.

Communications is recognized as the most trusted and knowledgeable source of industry information for today’s computing professional.

Communications brings its readership in-depth coverage of emerging areas of computer science, new trends in information technology, and practical applications. Industry leaders use Communications as a platform to present and debate various technology implications, public policies, engineering challenges, and market trends. The prestige and unmatched reputation that Communications of the ACM

enjoys today is built upon a 50-year commitment to high-quality editorial content and a steadfast dedication to advancing the arts, sciences, and applications of information technology.

P
LEASE RECYCL
E

T
HIS M A GAZIN
E

ACM, the world’s largest educational and scientific computing society, delivers resources that advance computing as a science and profession. ACM provides the computing field’s premier Digital Library and serves its members and the computing profession with leading-edge publications, conferences, and career resources.

Executive Director and CEO

Vicki L. Hanson

Deputy Executive Director and COO

Patricia Ryan

Director, Office of Information Systems

Wayne Graves

Director, Office of Financial Services

Darren Ramdin

Director, Office of SIG Services

Donna Cappo

Director, Office of Publications

Scott E. Delman

ACM COUNCIL

President

Cherri M. Pancake

Vice-President

Elizabeth Churchill

Secretary/Treasurer

Yannis Ioannidis

Past President

Alexander L. Wolf

Chair, SGB Board

Jeff Jortner

Co-Chairs, Publications Board

Jack Davidson and Joseph Konstan

Members-at-Large

Gabriele Anderst-Kotis; Susan Dumais;

Claudia Bauzer Mederios; Elizabeth D. Mynatt;

Pamela Samuelson; Theo Schlossnagle;

Eugene H. Spafford

SGB Council Representatives

Sarita Adve; Jeanna Neefe Matthews

BOARD CHAIRS

Education Board

Mehran Sahami and Jane Chu Prey

Practitioners Board

Terry Coatta and Stephen Ibaraki

REGIONAL COUNCIL CHAIRS

ACM Europe Council

Chris Hankin

ACM India Council

Abhiram Ranade

ACM China Council

Wenguang Chen

PUBLICATIONS BOARD

Co-Chairs

Jack Davidson; Joseph Konstan

Board Members

Phoebe Ayers; Ed ward A. Fox;

Chris Hankin; Xiang-Yang Li;

Sue Moon; Michael L. Nelson;

Sharon Oviatt; Eugene H. Spafford;

Stephen N. Spencer; Robert Walker;

Julie R. Williamson

ACM U.S. Public Policy Office

Adam Eisgrau, Director of Global Policy and Public Affairs

1701 Pennsylvania Ave N W, Suite 300, Washington, DC 20006 USA T [phone]; F [phone]

Computer Science Teachers Association

Jake Baskin Executive Director

STAFF

DIRECTOR OF PUBLICATIONS

Scott E. Delman [e-mail]

Executive Editor

Diane Crawford

Managing Editor

Thomas E. Lambert

Senior Editor

Andrew Rosenbloom

Senior Editor/News

Lawrence M. Fisher

Web Editor

David Roman

Rights and Permissions

Deborah Cotton

Editorial Assistant

Jade Morris

Art Director

Andrij Borys

Associate Art Director

Margaret Gray

Assistant Art Director

Mia Angelica Balaquiot

Production Manager

Bernadette Shade

Advertising Sales Account Manager

Ilia Rodriguez

Columnists

David Anderson; Michael Cusumano;

Peter J. Denning; Mark Guzdial;

Thomas Haigh; Leah Hoffmann; Mari Sako;

Pamela Samuelson; Marshall Van Alstyne

CONTACT POINTS

Copyright permission

[e-mail]

Calendar items

[e-mail]

Change of address

[e-mail]

Letters to the Editor

[e-mail]

WEBSITE

http://cacm.acm.org

WEB BOARD Chair

James Landay

Board Members

Marti Hearst; Jason I. Hong;

Jeff Johnson; Wendy E. MacKay

AUTHOR GUIDELINES

http://cacm.acm.org/about- communications/author-center

ACM ADVERTISING DEPARTMENT

2 Penn Plaza, Suite 701, New York, N Y

10121-0701

T [phone]

F [phone]

Advertising Sales Account Manager

Ilia Rodriguez [e-mail]

Media Kit [e-mail]

Association for Computing Machinery (ACM)

2 Penn Plaza, Suite 701

New York, N Y 10121-0701 USA T [phone]; F [phone]

EDITORIAL BOARD

EDITOR-IN-CHIEF

Andrew A. Chien [e-mail]

Deputy to the Editor-in-Chief

Lihan Chen cacm.deputy.to. [e-mail]

SENIOR EDITOR

Moshe Y. Vardi

NEWS Co-Chairs

William Pulleyblank and Marc Snir

Board Members

Monica Divitini; Mei Kobayashi;

Michael Mitzenmacher; Rajeev Rastogi;

François Sillion

VIEWPOIN TS Co-Chairs

Tim Finin; Susanne E. Hambrusch;

John Leslie King; Paul Rosenbloom

Board Members

Stefan Bechtold; Michael L. Best; Judith Bishop;

Andrew W. Cross; Mark Guzdial; Haym B. Hirsch;

Richard Ladner; Carl Landwehr; Beng Chin Ooi;

Francesca Rossi; Loren Terveen;

Marshall Van Alstyne; Jeannette Wing;

Susan J. Winter

PRAC TICE Co-Chairs

Stephen Bourne and Theo Schlossnagle

Board Members

Eric Allman; Samy Bahra; Peter Bailis;

Terry Coatta; Stuart Feldman; Nicole Forsgren;

Camille Fournier; Jessie Frazelle;

Benjamin Fried; Tom Killalea; Tom Limoncelli;

Kate Matsudaira; Marshall Kirk McKusick;

Erik Meijer; George Neville-Neil;

Jim Waldo; Meredith Whittaker

CONTRIBUTED ARTICLES Co-Chairs

James Larus and Gail Murphy

Board Members

William Aiello; Robert Austin; Kim Bruce;

Alan Bundy; Peter Buneman; Carl Gut win;

Yannis Ioannidis; Gal A. Kaminka;

Ashish Kapoor; Kristin Lauter; Igor Markov;

Bernhard Nebel; Lionel M. Ni; Adrian Perrig;

Marie-Christine Rousset; Krishan Sabnani;

m.c. schraefel; Ron Shamir; Alex Smola;

Josep Torrellas; Sebastian Uchitel;

Hannes Werthner; Reinhard Wilhelm

RESEARCH HIGHLIGHTS Co-Chairs

Azer Bestavros and Shriram Krishnamurthi

Board Members

Martin Abadi; Amr El Abbadi; Sanjeev Arora;

Michael Backes; Maria-Florina Balcan;

David Brooks; Stuart K. Card; Jon Crowcroft;

Alexei Efros; Bryan Ford; Alon Halevy;

Gernot Heiser; Takeo Igarashi; Sven Koenig;

Greg Morrisett; Tim Roughgarden;

Guy Steele, Jr.; Robert Williamson;

Margaret H. Wright; Nicholai Zeldovich;

Andreas Zeller

SPECIAL SECTIONS Co-Chair

Sriram Rajamani

Board Members

Tao Xie; Kenjiro Taura; David Padua

ACM Copyright Notice

Copyright © 2018 by Association for Computing Machinery, Inc. (ACM).

Permission to make digital or hard copies of part or all of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and full citation on the first page. Copyright for components of this work owned by others than ACM must be honored. Abstracting with credit is permitted. To copy other wise, to republish, to post on servers, or to redistribute to lists, requires prior specific permission and/or fee. Request permission to publish from [e-mail] or fax [phone].

For other copying of articles that carry a code at the bottom of the first or last page or screen display, copying is permitted provided that the per-copy fee indicated in the code is paid through the Copyright Clearance Center; www.copyright.com.

Subscriptions

An annual subscription cost is included in ACM member dues of $99 ($40 of which is allocated to a subscription to

Communications); for students, cost is included in $42 dues ($20 of which is allocated to a Communications

subscription). A nonmember annual subscription is $269.

ACM Media Advertising Policy

Communications of the ACM and other ACM Media publications accept advertising in both print and electronic formats. All advertising in ACM Media publications is at the discretion of ACM and is intended to provide financial support for the various activities and services for ACM members.

Current advertising rates can be found by visiting http://www.acm-media.org or by contacting ACM Media Sales at [phone].

Single Copies

Single copies of Communications of the ACM are available for purchase. Please contact [e-mail].

COMMUNICATIONS OF THE ACM

(ISSN 0001-0782) is published monthly by ACM Media, 2 Penn Plaza, Suite 701, New York, N Y 10121-0701. Periodicals postage paid at New York, NY 10001, and other mailing offices.

POSTMASTER

Please send address changes to

Communications of the ACM

2 Penn Plaza, Suite 701

New York, N Y 10121-0701 USA Printed in the USA.