Insert the specified value into this set if a key equivalent to value does not already exist in this set; otherwise, if a key equivalent to value already exists in this set, this method has no effect. Return a pair whose first member is an iterator referring to the (possibly newly inserted) value_type object in this set that is equivalent to value, and whose second member is true if a new value was inserted, and false if the key was already present. This method requires that the (template parameter) type KEY be copy-insertable (see {Requirements on KEY}).