code: func cmdSetTag(params Parameters) *subcommands.Command {
  	return &subcommands.Command{
  		UsageLine: "set-tag <package or package prefix> -tag=key:value [options]",
  		ShortDesc: "tags package of a specific version",
  		LongDesc:  "Tags package of a specific version",
  		CommandRun: func() subcommands.CommandRun {
  			c := &setTagRun{}
  			c.registerBaseFlags()
  			c.tagsOptions.registerFlags(&c.Flags)
  			c.clientOptions.registerFlags(&c.Flags, params, withoutRootDir)
  			c.Flags.StringVar(&c.version, "version", "<version>",
  				"Package version to resolve. Could also be itself a tag or ref")
  			return c
  		},
  	}
  }
docs: ////////////////////////////////////////////////////////////////////////////////
  // 'set-tag' subcommand.