def [] index
      if index.is_a? Range
        n = index.size
        extract_band index.begin, n: n
      elsif index.is_a? Numeric
        extract_band index
      else
        raise Vips::Error, "[] index is not range or numeric."
      end
    end
def [] index
      if index.is_a? Range
        n = index.size
        extract_band index.begin, n: n
      elsif index.is_a? Numeric
        extract_band index
      else
        raise Vips::Error, "[] index is not range or numeric."
      end
    end
Fetch bands using a number or a range

 @param index [Numeric, Range] extract these band(s)
 @return [Image] extracted band(s)