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[2] C. C. Green: Application of Theorem Proving to Problem Solving. Proc. 1st Int. Joint Conf. on Artificial Intelligence, May 1969.

[3] J. McCarthy P. Hayes: Some Philosophical Problems from the Standpoint of Artificial Intelligence. In: Machine Intelligence 4, 1969.

[4] N. J. Nilsson: Problem-Solving Methods in Artifical Intelligence. McGraw-Hill, 1971.