def coerce_positions *positions
      if positions.size == 1
        case positions.first
        when Integer
          positions.first
        when Range
          size.times.to_a[positions.first]
        else
          raise ArgumentError, 'Unkown position type.'
        end
      else
        positions
      end
    end
def coerce_positions *positions
      if positions.size == 1
        case positions.first
        when Integer
          positions.first
        when Range
          size.times.to_a[positions.first]
        else
          raise ArgumentError, 'Unkown position type.'
        end
      else
        positions
      end
    end
coerce ranges, integers and array in appropriate ways