a value to be transformed. If, for example, top=10, then values 0 thru 10 are left unchanged, values 11 thru 19 are mapped to 9 thru 1 respectively, and 20 thru 30 are mapped to 0 thru 10, and so on. Transforms linear counting (0, 1, 2, ...) into a back-and-forth counting (0, 1, 2, 1, 0, ...) from 0 to a specified upper bound. upper bound