a counter-question, rob: how would you handle the addition of 2 numbers between 90 and 100, in your model? there are no possible outputs (180-200) within the input range (90-100). if, as i believe you suggest, the output range must match the input range, you would have to cast the inputs to match the output range. but the input and output ranges are disjunct one solution would be to cast the inputs to the union of the output and input ranges (90-200), then perform the addition. of course, you would then want to cast the result back to the real output range of 180-200 that seems cumbersome to me, but more importantly, you lose the opportunity for letting the compiler provide valuable validation of your design in my field (digital electronics) at least, there is little use for operators that are unable to produce results outside of the input range examples of trivial operations that do not adhere to that limitation include: - adding two 5 bit integers (result must be stored in a 6 bit integer) - multiplying an 11 bit integer by a 16 bit integer (result must be stored in a 27 bit integer) - shifting an 11 bit integer up by 5 positions (result must be stored in a 16 bit integer) granted, my field is somewhat specialized, but i don't think that the case where the minimum and maximum voltage of both your batteries happens to match the minimum and maximum voltage of your input is all that common. is it really that practical to hook two 2V-5V batteries in serial up to a 2V-5V input? more common than a 4V-10V input? or even a 2V-7V input? i mean: the latter would at least allow you to fully charge one of the batteries without destroying the system regards, /michael toksvig