The original paper was published in 1992 (<a href="http:&#x2F;&#x2F;dl.acm.org&#x2F;citation.cfm?id=157525" rel="nofollow">http:&#x2F;&#x2F;dl.acm.org&#x2F;citation.cfm?id=157525</a>) but doesn&#x27;t seem to be freely accessible anywhere in PDF format.