anchor.diff:  
   //
   [<CompiledName("FoldBackIndexed")>]
  -let foldiBack (folder : int -> 'T -> 'State -> 'State) (resizeArray : ResizeArray<'T>) state =
  +let foldiBack (folder : int -> 'T -> 'State -> 'State) (resizeArray : ResizeArray<'T>) state : 'State =
       // Preconditions
       checkNonNull "resizeArray" resizeArray
   
  
positive.code:     let mutable state = state
      let count = resizeArray.Count
      for i = 0 to count - 1 do
          state <- folder.Invoke (i, state, resizeArray.[i])
      state
  
  /// <summary>
  
negative.code:         index <- index + 1
      result
  
  /// <summary>
  /// Applies the given function to successive elements, returning the first
  /// result where function returns <c>Some(x)</c> for some x. If the function
  /// never returns <c>Some(x)</c>, raises KeyNotFoundException.
  /// </summary>
  [<CompiledName("Pick")>]
  let pick (picker : 'T -> 'U option) (resizeArray : ResizeArray<'T>) : 'U =
      // Preconditions
  