struct FlattenSequence.Iterator A type that provides the sequence's iteration interface and encapsulates its iteration state. Inheritance IteratorProtocol View Protocol Hierarchy → Associated Types Element = Base.Element.Element The type of element traversed by the iterator. Import import Swift Instance Methods mutating func next() Advances to the next element and returns it, or nil if no next element exists. Once nil has been returned, all subsequent calls return nil. Precondition: next() has not been applied to a copy of self since the copy was made. Declaration mutating func next() -> FlattenSequence<Base>.Iterator.Element?