def parse_attribute
      ns_id, name_id, val_str_id, flags, val = @io.read(4*5).unpack("V*")
      key = @strings[name_id]
      unless ns_id == 0xFFFFFFFF
        ns = @strings[ns_id] 
        prefix = ns.sub(/.*\//,'')
        unless @ns.include? ns
          @ns << ns
          @doc.root.add_namespace(prefix, ns)
        end
        key = "#{prefix}:#{key}"
      end
      value = convert_value(val_str_id, flags, val)
      return key, value
    end
def parse_attribute
      ns_id, name_id, val_str_id, flags, val = @io.read(4*5).unpack("V*")
      key = @strings[name_id]
      unless ns_id == 0xFFFFFFFF
        ns = @strings[ns_id] 
        prefix = ns.sub(/.*\//,'')
        unless @ns.include? ns
          @ns << ns
          @doc.root.add_namespace(prefix, ns)
        end
        key = "#{prefix}:#{key}"
      end
      value = convert_value(val_str_id, flags, val)
      return key, value
    end
parse attribute of a element