code: func (cl *CommandLine) AddFlag(flag CommandLineFlag) {
  	if strings.HasPrefix(flag.Flag, "-") {
  		panic("flag must not begin with '-'")
  	}
  	for _, f := range cl.Flags {
  		if f == flag {
  			return
  		}
  	}
  	cl.Flags = append(cl.Flags, flag)
  }
docs: // AddFlag adds an interpreter flag to cl if it's not already present.