“One way to protect the functioning of reputation systems is to inject sources that are as close as possible to ground truth. A single fact that is certainly true can invalidate any number of sources that are only somewhat trustworthy, if those sources disseminate information contrary to the known fact. In many countries, notaries function as sources of ground truth to maintain the integrity of legal and real-estate information; they are usually disinterested third parties in any transaction and are licensed by governments or professional societies.”