def prop atom
      val, format, nitems = prop_raw atom
      case format
      when 32; val.unpack("l!#{nitems}")
      when 16; val.unpack("s#{nitems}")
      when 8; val[0, nitems]
      when 0; nil
      end
    end
def prop atom
      val, format, nitems = prop_raw atom
      case format
      when 32; val.unpack("l!#{nitems}")
      when 16; val.unpack("s#{nitems}")
      when 8; val[0, nitems]
      when 0; nil
      end
    end
window property getter with easy way for XGetWindowProperty
 which return nil, if the specified property name does not exist,
 a String or a Array of Number