anchor.diff:      for i = 0 to length arr - 1 do 
           let x = arr.[i] 
           if f x then res1.Add(x) else res2.Add(x)
  -    res1, res2+    res1, res2
  +
  +
  +let inline private indexNotFound () =
  +    keyNotFound "An index satisfying the predicate was not found in the collection"
  +
  +//
  +[<CompiledName("FindIndexIndexed")>]
  +let findIndexi f (arr: ResizeArray<_>) =
  +    let rec go n = if n >= length arr then indexNotFound() elif f n arr.[n] then n else go (n+1)
  +    go 0
  +
  
positive.code:     result
  
  //
  [<CompiledName("Iterate")>]
  let iter (action : 'T -> unit) (resizeArray : ResizeArray<'T>) =
      // Preconditions
      checkNonNull "resizeArray" resizeArray
  
negative.code: let scanBack f (arr : ResizeArray<'T>) acc = 
      let arrn = length arr
      scanBackSub f arr 0 (arrn - 1) acc
          
  //
  [<CompiledName("TryFindIndexIndexed")>]
  