Link to mentioned paper <a href="http:&#x2F;&#x2F;www.cs.cmu.edu&#x2F;~15-440&#x2F;READINGS&#x2F;borg-1989.pdf" rel="nofollow">http:&#x2F;&#x2F;www.cs.cmu.edu&#x2F;~15-440&#x2F;READINGS&#x2F;borg-1989.pdf</a>