You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: timezone_transition_definition.rb
    path of file: ./repos/brew/Library/Homebrew/vendor/bundle-standalone/ruby/2.3.0/gems/tzinfo-1.2.5/lib/tzinfo
    
    the code of the file until where you have to start completion: module TZInfo
  # A TimezoneTransition defined by as integer timestamp, as a rational to
  # create a DateTime or as both.
  #
  # @private 
  class TimezoneTransition
    
    def at
      # Thread-safety: It is possible that the value of @at may be calculated
      # multiple times in concurrently executing threads. It is not worth the
      # overhead of locking to ensure that @at is only calculated once.
      
      unless @at
        result = unless @denominator
          TimeOrDateTime.new(@numerator_or_time)
        else
          r = RubyCoreSupport.rational_new!(@numerator_or_time, @denominator)
          dt = RubyCoreSupport.datetime_new!(r, 0, Date::ITALY)
          TimeOrDateTime.new(dt)
        end