def last
      return nil if ! @map.last
      if @map.length == 1
        left = 0
      else
        left = @map[@map.length-2][0]+1
      end
      [@map.length-1,@map.last[1],left,@map.last[0]]
    end
def last
      return nil if ! @map.last
      if @map.length == 1
        left = 0
      else
        left = @map[@map.length-2][0]+1
      end
      [@map.length-1,@map.last[1],left,@map.last[0]]
    end
For the final region, return an array of the form [index, value, left, right]