“…They have been used extensively to understand the nature of intelligence--in cognitive modeling, in the study of problem-solving systems, and in the study of learning systems [2,44,45,62,73,74,92]. They have also been used extensively to develop large expert systems spanning a variety of applications in areas including computer-aided design, medicine, configuration tasks, oil exploration [11, 14,39,41,42,55,56,81]. Production-system programs, however, are computation intensive and run quite slowly.…”