   }
 }
 
-extension Concatenation : BidirectionalCollection
+extension Concatenation: BidirectionalCollection
 where Base1: BidirectionalCollection, Base2: BidirectionalCollection {
   public func index(before i: Index) -> Index {
     assert(i != startIndex, "Can't advance before startIndex")

  }
}

extension RotatedCollection : RandomAccessCollection
where Base: RandomAccessCollection {}

extension Collection {

/// of a collection to `MutableCollectionAlgorithms` to get these customization
/// points to be used from other algorithms defined on
/// `MutableCollectionAlgorithms`.
public protocol MutableCollectionAlgorithms: MutableCollection
where SubSequence: MutableCollectionAlgorithms
{
  /// Rotates the elements of the collection so that the element
  /// at `middle` ends up first.
