Insert the specified value into this set (in amortized constant time if the specified hint is a valid immediate successor to value) if a key equivalent to value does not already exist in this set; otherwise, this method has no effect. value is left in a valid but unspecified state. Return an iterator referring to the (possibly newly inserted) value_type object in this set that is equivalent to value. If hint is not a valid immediate successor to value, this operation has O[log(N)] complexity, where N is the size of this set. This method requires that the (template parameter) type KEY be move‐insertable (see {Requirements on KEY}). The behavior is undefined unless hint is an iterator in the range [begin() .. end()] (both endpoints included).