Rüdiger Schätz. Delete Operations and Horn Formulas. In Egon Börger, Hans Kleine Büning, Michael M. Richter, editors, CSL 88, 2nd Workshop on Computer Science Logic, Duisburg, Germany, October 3-7, 1988, Proceedings. Volume 385 of Lecture Notes in Computer Science, pages 329-343, Springer, 1988.