The term "arity" is primarily used with reference to functions of the form f : V → S, where V ⊂ Sn, and S is some set. Such a function is often called an operation on S, and n is its arity. Arities greater than 2 are seldom encountered in mathematics, except in specialized areas, and arities greater than 3 are seldom encountered in theoretical computer science. In computer programming there is often a syntactical distinction between operators and functions; syntactical operators usually have arity 0, 1 or 2. In mathematics, depending on the branch, arity may be called type, adicity or rank.