   let w = "world!"
   let hw = concatenate(h, w)
   expectEqual("Hello, world!", String(hw))
+  
+  let run = (1...).prefix(10).followed(by: 20...)
+  expectEqual(Array(run.prefix(20)), Array(1...10) + (20..<30))
 }
 
 suite.test("stablePartition") {

}

extension Concatenation : BidirectionalCollection
  where C1: BidirectionalCollection, C2: BidirectionalCollection
{
  public func index(before i: Index) -> Index {
    assert(i != startIndex, "Can't advance before startIndex")

}

extension Concatenation : RandomAccessCollection
  where Base1: RandomAccessCollection, Base2: RandomAccessCollection
{
  public func index(_ i: Index, offsetBy n: Int) -> Index {
    if n == 0 { return i }
