It may seem that the switch to exact isotope mass from integer isotope number might be difficult to implement, requiring access to a complete table of isotopes, but that is not the case. It turns out that we can use integer isotope mass numbers provided we take account of just four anomalies: O, Cr, Mo, and 175 Lu. These four isotopes are the only ones that have exact masses slightly below their element's atomic weight even though their mass number is above it. For example, the exact mass of O is 15.994, which is below the element's atomic weight of 15.999, even though its mass number ( ) is higher. In practice, this is no problem. We simply use integer isotope numbers, but reduce these four by 0.1 for the purpose of setting priorities. For example, since the atomic weight of oxygen is 15.999, when O is compared to O, there is no problem --17 > 15.999. But when O is specified, we use 15.9 instead of its actual value of 15.994. This allows 16 O to have the required lower priority than "O" itself, with atomic weight 15.999. No other isotopes have this problem, and we can just use their unadjusted integer mass number as a surrogate for isotopic mass. In this way, there is never a need to check a table of exact isotope masses.