def emitIzPackXML(xm)
      # raise "xm must be an Builder::XmlMarkup object, but is #{xm.class}" if xm.class != Builder::XmlMarkup
      xm.pack(@attributes) {
        xm.description(@description)
        @files.each{ |src, dest|  xm.singlefile('src'=> src, 'target' =>dest) }
      }
    end
def emitIzPackXML(xm)
      # raise "xm must be an Builder::XmlMarkup object, but is #{xm.class}" if xm.class != Builder::XmlMarkup
      xm.pack(@attributes) {
        xm.description(@description)
        @files.each{ |src, dest|  xm.singlefile('src'=> src, 'target' =>dest) }
      }
    end
collect the XML representation for the pack using an XMLMarkup object