```swift var numbers = Array(1...15) // Find the indices of all the even numbers let indicesOfEvens = numbers.subranges(where: { $0.isMultiple(of: 2) }) // Perform an operation with just the even numbers let sumOfEvens = numbers[indicesOfEvens].reduce(0, +) // sumOfEvens == 56 ```

```swift extension Collection { /// Accesses a view of this collection with the elements at the given /// indices. /// /// - Complexity: O(1) public subscript(subranges: RangeSet<Index>) -> DiscontiguousSlice<Self> { get } } extension MutableCollection { /// Accesses a mutable view of this collection with the elements at the /// given indices. /// /// - Complexity: O(1) to access the elements, O(m) to mutate the /// elements at the positions in subranges, where m is the number of /// elements indicated by subranges. public subscript(subranges: RangeSet<Index>) -> DiscontiguousSlice<Self> { get set } } /// A collection wrapper that provides access to the elements of a collection, /// indexed by a set of indices. public struct DiscontiguousSlice<Base: Collection>: Collection { /// The collection that the indexed collection wraps. public var base: Base { get set } /// The set of index ranges that are available through this indexing /// collection. public var subranges: RangeSet<Base.Index> { get set } /// A position in an DiscontiguousSlice. struct Index: Comparable { // ... } public var startIndex: Index { get } public var endIndex: Index { set } public subscript(i: Index) -> Base.Element { get } public subscript(bounds: Range<Index>) -> Self { get } } ```