code: func (u *User) Mention() string {
  
  	if u.UserName != "" {
  		return "@" + u.UserName
  	}
  
  	name := u.FirstName
  	if u.LastName != "" {
  		name += " " + u.LastName
  	}
  
  	return name
  }
docs: // Mention returns @username if available. First + Last name otherwise