This is only about transliteration (tr///), not rules in general. So you are only matching a fixing set of strings at a certain position. If one string is the prefix of another, the longer is preferred. If there are two identical match strings, the replacement corresponding to the first is used.
My original post was confused and confusing. Brad -- Among the words spoken by great generals, there are some that were said offhandedly. One should not receive these words in the same manner, however. -- Hagakure http://bereft.net/hagakure/