docs: def do_ls(self, subcmd, opts, folder=""):
          """${cmd_name}: list messages in the specified folder
  
          ${cmd_usage}
          ${cmd_option_list}
          SINCE can be used with epoch times, for example: 
  
            md ls -s $(date '+%s')
          """
          client = MdClient(self.maildir, filesystem=self.filesystem)
          client.ls(
              foldername = folder, 
              stream = self.stdout, 
              reverse = getattr(opts, "reverse", False),
              grep = getattr(opts, "grep", None),
              field = getattr(opts, "field", None),
              since = float(getattr(opts, "since", -1))
              )