 
   @inlinable // protocol-only
   public func index(_ i: Index, offsetBy n: Int) -> Index {
+    return _index(i, offsetBy: n)
+  }
+
+  @inlinable // protocol-only
+  internal func _index(_ i: Index, offsetBy n: Int) -> Index {
     if n >= 0 {
       return _advanceForward(i, by: n)
     }


  @inlinable // protocol-only
  public func distance(from start: Index, to end: Index) -> Int {
    var start = start
    var count = 0



  @inlinable // protocol-only
  public func index(
    _ i: Index, offsetBy n: Int, limitedBy limit: Index
  ) -> Index? {
    return _index(i, offsetBy: n, limitedBy: limit)
  }

  @inlinable // protocol-only
  internal func _index(
    _ i: Index, offsetBy n: Int, limitedBy limit: Index
  ) -> Index? {
    if n >= 0 {
