Combine 2 sequence in order to perform combination mapping (like for inside for)

Combine2Sequence.swift

public struct Combine2Sequence<Base1: Sequence, Base2: Sequence>: Sequence {
    private let base1: Base1
    private let base2: Base2

    public init(_ base1: Base1, _ base2: Base2) {
        self.base1 = base1
        self.base2 = base2
    }

    public func makeIterator() -> Iterator {
        Iterator(base1Iterator: base1.makeIterator(), base2: base2)
    }

    public struct Iterator: IteratorProtocol {
        private var base1Iterator: Base1.Iterator
        private var base2: Base2
        private var base2Iterator: Base2.Iterator?
        private var currentElement1: Base1.Element?

        fileprivate init(base1Iterator: Base1.Iterator, base2: Base2) {
            self.base1Iterator = base1Iterator
            self.base2 = base2
            self.base2Iterator = base2.makeIterator()
            self.currentElement1 = self.base1Iterator.next()
        }

        public mutating func next() -> (Base1.Element, Base2.Element)? {
            if let element1 = currentElement1, let element2 = base2Iterator?.next() {
                return (element1, element2)
            } else if let element1 = base1Iterator.next() {
                currentElement1 = element1
                self.base2Iterator = base2.makeIterator()
                if let element2 = base2Iterator?.next() {
                    return (element1, element2)
                }
            }
            return nil
        }
    }
}

func combine<Base1: Sequence, Base2: Sequence>(_ s1: Base1, _ s2: Base2) -> Combine2Sequence<Base1, Base2> {
    Combine2Sequence(s1, s2)
}

let combinedSequence = combine([1,2,3], [10,100,1000])
combinedSequence.map(+) == [11,101,1001,12,102,1002,13,103,1003] // true

let combinedSequence1 = combine([2,5,10], [8,10,11])
combinedSequence1.map(*) == [16,20,22,40,50,55,80,100,110] // true