Implementing a Collection.removingIndices(_:)

Using Swift
23

Yes indeed, in fact all of them have big trouble dealing with arrays with more than 1000 elements.
The only faster implementations are @nnnnnnnn version:

Iterative copying 
extension Collection {
    func removingIndices<S: Sequence>(_ indicesToRemove: S) -> [Element]
        where S.Element == Index
    {
        var result: [Element] = []
        var low = startIndex
        for high in indicesToRemove {
            result.append(contentsOf: self[low..<high])
            low = index(after: high)
        }
        result.append(contentsOf: self[low...])
        return result
    }
}

and @vadian version:

It's all about partitions 
extension RangeReplaceableCollection where Self: MutableCollection, Index == Int {

    mutating func remove(at indexes : IndexSet) {
        guard var i = indexes.first, i < count else { return }
        var j = index(after: i)
        var k = indexes.integerGreaterThan(i) ?? endIndex
        while j != endIndex {
            if k != j { swapAt(i, j); formIndex(after: &i) }
            else { k = indexes.integerGreaterThan(k) ?? endIndex }
            formIndex(after: &j)
        }
        removeSubrange(i...)
    }
}

@dabrahams the "Iterative copying" version is not O(n²), isn't it? If it is, then I don't understand how it is much faster than the other (bad) implementations.
Thank you for your amazing WWDC presentation :ok_hand: