def fetch_pull_request_id
      if positive_integer_string? ENV['TRAVIS_PULL_REQUEST']
        ENV['TRAVIS_PULL_REQUEST'].to_i
      elsif positive_integer_string? ENV['CIRCLE_PR_NUMBER']
        ENV['CIRCLE_PR_NUMBER'].to_i
      elsif positive_integer_string? ENV['BITRISE_PULL_REQUEST']
        ENV['BITRISE_PULL_REQUEST'].to_i
      elsif positive_integer_string? ENV['ghprbPullId']
        ENV['ghprbPullId'].to_i
      end
    end
def fetch_pull_request_id
      if positive_integer_string? ENV['TRAVIS_PULL_REQUEST']
        ENV['TRAVIS_PULL_REQUEST'].to_i
      elsif positive_integer_string? ENV['CIRCLE_PR_NUMBER']
        ENV['CIRCLE_PR_NUMBER'].to_i
      elsif positive_integer_string? ENV['BITRISE_PULL_REQUEST']
        ENV['BITRISE_PULL_REQUEST'].to_i
      elsif positive_integer_string? ENV['ghprbPullId']
        ENV['ghprbPullId'].to_i
      end
    end
@!attribute [r] pull_request_id
   @return [Integer, nil] pull request id or nil
 Build pull request information object from environment variables

 @overload initialize
   @example without user defined block
     env_pull = EnvPullRequest::Base.new

   @return [Base] pull request information object

 @overload initialize(&block)
   @example with user defined block
     env_pull =
       EnvPullRequest::Base.new do
         if NaturalNumberString
              .positive_integer_string? ENV['PULL_REQUEST_ID']
           ENV['PULL_REQUEST_ID'].to_i
         end
       end

   @yield user defined block
   @return [Base] pull request information object
 Fetch pull request id from environment variables

 travis-ci.org:
   ENV['TRAVIS_PULL_REQUEST']
 circleci.com:
   ENV['CIRCLE_PR_NUMBER']
 bitrise.io:
   ENV['BITRISE_PULL_REQUEST']
 Jenkins GitHub pull request builder plugin:
   ENV['ghprbPullId']

 @return [Integer, nil] pull request id or nil

 @see TestHelper.stash_env_pull_request
 @see TestHelper.restore_env_pull_request

 @see http://docs.travis-ci.com/user/environment-variables/#Default-Environment-Variables
   Environment Variables - Travis CI
 @see https://circleci.com/docs/environment-variables#building-pull-requests-that-come-from-forks
   Environment variables - CircleCI
 @see https://devcenter.bitrise.io/faq/available-environment-variables/
   Environment variables - Bitrise
 @see https://wiki.jenkins-ci.org/display/JENKINS/GitHub+pull+request+builder+plugin#GitHubpullrequestbuilderplugin-EnvironmentVariables
   GitHub pull request builder plugin - Jenkins - Jenkins Wiki