anchor.diff:              ret.Array.[subidx] <- nodeToInsert :> obj
               ret
   
  -        member internal this.ArrayFor<'a> i =
  +        member internal this.ArrayFor i =
               if i >= 0 && i < count then
                   if i >= this.TailOff() then tail else
                       let mutable node = root
  
positive.code:             else raise Exceptions.OutOfBounds
  
  
          member internal this.EditableArrayFor<'a> i =
              if i >= 0 && i < count then
                  if i >= this.TailOff() then tail else
                      let mutable node = root
  
negative.code:             ret.Array.[subidx] <- nodeToInsert :> obj
              ret
  
          member internal this.ArrayFor i =
              if i >= 0 && i < count then
                  if i >= tailOff then tail else
                      let mutable node = root
  