What does $1 ++ $0 mean in the closure for Swift?

Question

I was looking at the code for how to remove duplicates from a list and I came upon some syntax I am unfamiliar with. What does $1 ++ $0 mean?

extension List where Element: Hashable {

    func removeDuplicates() -> List {
        var set = Set<Element>()
        let list = reduce(List()) {
            guard !set.contains($1) else { return $0 }
            set.insert($1)
            return $1 ++ $0
        }
        return list.reversed()
    }
}

Answer 1

$0 is the first parameter passed into the closure.

$1 is the second parameter.

++ is a custom infix operator

infix operator ++

private func ++<Element>(element: Element, list: ListNode<Element>) -> ListNode<Element> {
    return list.insert(element: element)
}

It will insert the element on the right to list on right of the operator