a.swift

public extension Sequence {
    public func mySequence() -> AnySequence<Element> {
        return AnySequence { () -> AnyIterator<Element> in
            var iterator = self.makeIterator()
            return AnyIterator {
                return iterator.next()
            }
        }
    }
}


func f() {
    let b = [1, 2, 3, 4]
    let s = b.mySequence().drop { $0 < 3 }
    let i1 = Array(s) // [3, 4]
    let i2 = Array(s) // [3]
    // why does i1 not equal i2? and when you just do `let s = b.mySequence()` or `b.mySequence().lazy` without the drop, it works fine
}