This question concerns computing subtraction using a production system. Assume that WM initially contains information to deal with individual digits in the following form: (digitMinus top: n bot: m ans: k borrow: b), where n and m are any digits, and if n ≥ m, then k is n − m and b is 0, else k is 10 + n − m and b is 1. For example, (digitMinus top: 7 bot: 3 ans: 4 borrow: 0) would be in WM, as would (digitMinus top: 3 bot: 7 ans: 6 borrow: 1). The working memory also specifies the first and second arguments of a subtraction problem (the subtrahend and minuend): (topNum pos: i digit: d left: j) and (botNum pos: i digit: d left: j), where d is a digit, and i and j are indices indicating the current position of the digit and its neighbor to the left, respectively