I considered defining the comparison of txids in terms of integers; however, I am concerned that this definition may be ambiguous when applied to a variety of languages and platforms without a similar amount of explanation as currently exists. For example, if a web wallet uses an API to receive transaction information, this is traditionally expressed in terms tx id strings rather than 256-bit integers. My intent is that wallets can implement the algorithm however they wish, but should ensure that their output is compliant with the BIP definition. IMHO the algorithm stated in the BIP should target test cases rather than implementation, and should leave as little room for ambiguity as possible.