   ///
   /// Should never be called, since this collection is always empty.
   public subscript(position: Index) -> Element {
+    // TODO: swift-3-indexing-model: tests for traps.
     _preconditionFailure("Index out of range")
   }
 

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


  /// 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")
  }

