def get(nickname)
      users.each do |user|
        if user.nickname == nickname.to_s
          return user
        end
      end

      nil
    end
def get(nickname)
      users.each do |user|
        if user.nickname == nickname.to_s
          return user
        end
      end

      nil
    end
Returns the user with nickname nil if no such user exists
 @param [String|Symbol] nickname
 @return [User|nil]