def get_access_token
      # get saved access token
      if ::File.exists?(ACCESS_TOKEN)
        @access_token = ::YAML.load_file(ACCESS_TOKEN)
      end
      # if access token is nil,
      # then get a new initial token
      if @access_token.nil?
        get_initial_token
        ::File.open(ACCESS_TOKEN, 'w') do |out|
          YAML.dump(@access_token, out)
        end
      end
    end
def get_access_token
      # get saved access token
      if ::File.exists?(ACCESS_TOKEN)
        @access_token = ::YAML.load_file(ACCESS_TOKEN)
      end
      # if access token is nil,
      # then get a new initial token
      if @access_token.nil?
        get_initial_token
        ::File.open(ACCESS_TOKEN, 'w') do |out|
          YAML.dump(@access_token, out)
        end
      end
    end
get the access token