indices_expression.swift

// This makes it possible to use an `Indices` in a collection's subscript.
// Since indices are generally their own slice type, this makes using
// the subsequence of an indices useful...
extension DefaultRandomAccessIndices : RangeExpression {
  public func relative<C>(to collection: C) -> Range<Element>
    where C : _Indexable, Element == C.Index
  {
    if isEmpty {
      return collection.startIndex ..< collection.startIndex
    }
    return first! ..< collection.index(after: last!)
  }
}

let a = [1, 2, 3, 1, 2, 3, 1, 2, 3]
let rev = a.reversed()
let revIndices = (rev.indices.dropFirst(2) as DefaultRandomAccessIndices).dropLast(2)
Array(rev[revIndices].reversed())    // [3, 1, 2, 3, 1]