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The ACM copyright notice on the 1988 and 1990 papers is:
Permission to copy without fee all or part of this material
The copies here are being distributed without any direct or indirect
commercial advantage so to my understanding they are being distributed
with the permission of the Association for Computing Machinery.
granted provided that the copies are not made or distributed
for direct commercial advantage, the ACM copyright notice and
title of the publication and its date appear, and notice is
given that copying is by permission of the Association for
Computing Machinery. To copy otherwise, or to republish,
requires a fee and/or specific permission.