A special kind of auxiliary function for algorithms is a predicate. Predicates are functions that return a Boolean value. They are often used to specify a sorting or a search criterion. Depending on their purpose, predicates are unary or binary. Note that not every unary or binary function that returns a Boolean value is a valid predicate. The STL requires that predicates always yield the same result for the same value. This rules out functions that modify their internal state when they are called.