Anonymity, adjective "anonymous", is derived from the Greek word ἀνωνυμία, anonymia, meaning "without a name" or "namelessness". In colloquial use, "anonymous" is used to describe situations where the acting person's name is unknown. Some writers have argued that namelessness, though technically correct, does not capture what is more centrally at stake in contexts of anonymity. The important idea here is that a person be non-identifiable, unreachable, or untrackable. Anonymity is seen as a technique, or a way of realizing, certain other values, such as privacy, or liberty. An important example for anonymity being not only protected, but enforced by law is probably the vote in free elections. In many other situations (like conversation between strangers, buying some product or service in a shop), anonymity is traditionally accepted as natural. There are also various situations in which a person might choose to withhold their identity. Acts of charity have been performed anonymously when benefactors do not wish to be acknowledged. A person who feels threatened might attempt to mitigate that threat through anonymity. A witness to a crime might seek to avoid retribution, for example, by anonymously calling a crime tipline. Criminals might proceed anonymously to conceal their participation in a crime. Anonymity may also be created unintentionally, through the loss of identifying information due to the passage of time or a destructive event. In certain situations, however, it may be illegal to remain anonymous. In the United States, 24 states have “stop and identify” statutes that requires persons detained to self-identify when requested by a law enforcement officer. In Germany, people have to indicate their names at the door of their homes. The term "anonymous message" typically refers to a message that does not reveal its sender. In many countries, anonymous letters are protected by law and must be delivered as regular letters. In mathematics, in reference to an arbitrary element (e.g., a human, an object, a computer), within a well-defined set (called the "anonymity set"), "anonymity" of that element refers to the property of that element of not being identifiable within this set. If it is not identifiable, then the element is said to be "anonymous."