   /// possible to advance indices.
   @warn_unused_result
   public func next(i: Index) -> Index {
+    // TODO: swift-3-indexing-model: tests for traps.
     fatalError("EmptyCollection can't advance indices")
   }
 

  /// possible to advance indices.
  @warn_unused_result
  public func advance(i: Index, by n: IndexDistance) -> Index {
    fatalError("EmptyCollection can't advance indices")
  }


  /// possible to advance indices.
  @warn_unused_result
  public func previous(i: Index) -> Index {
    // TODO: swift-3-indexing-model: tests for traps.
    fatalError("EmptyCollection can't advance indices")
  }

