enum Tree<Element> {
    case Leaf(Element)
    case Node(Element, [Tree])
}

func permutationTrees<Element>(withElements elements: [Element]) -> [Tree<Element>] {
    guard elements.count > 0 else {
        return []
    }
    guard elements.count > 1 else {
        return [.Leaf(elements[0])]
    }
    guard elements.count > 2 else {
        return [
            .Node(elements[0], [.Leaf(elements[1])]),
            .Node(elements[1], [.Leaf(elements[0])]),
        ]
    }
    // GenericCache(0x10000ef80): cyclic metadata dependency detected, aborting
    // Signal: SIGABRT (signal SIGABRT)
    var trees = [Tree<Element>]()
    for (index, element) in elements.enumerated() {
        var copy = elements
        copy.remove(at: index)
        let subtrees = permutationTrees(withElements: copy)
        trees.append(.Node(element, subtrees))
    }
    return trees
}