Young & O'Shea (1981) present a production system model of children's multi-column subtraction. The model has several appealing features: it deals with an approachable and interesting topic; it is quite small and can be contemplated as a whole; and it can be modified to examine different (incorrect) rule sets and the errors that follow.
Independently, and for different reasons, we each began a COGENT implementation of the model. The efforts merged and the result was ultimately successful. Furthermore, the use of an analogue buffer to display the problem and the model's working yielded a model which makes a very nice demonstration. However, the exercise raised a number of significant issues about COGENT. In particular, the published model used an OPS-style production system interpreter. The principal difficulty in developing the COGENT implementation concerned implementation of an appropriate conflict resolution mechanism. The conflict resolution procedure we eventually adopted is simple but not entirely satisfactory. We discuss the workings of the mechanism,the problems which remained unsolved, and the wider implications for COGENT.