code: func (bf *BFlags) AddString(name string, def string) *Flag {
  	flag := bf.addFlag(name, stringType)
  	if flag == nil {
  		return nil
  	}
  	flag.Value = def
  	return flag
  }
docs: // AddString adds a string flag to BFlags
  // Note, any error will be generated when Parse() is called (see Parse).