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: rack_env_mutation_test.rb
    path of file: ./repos/newrelic-ruby-agent/test/multiverse/suites/rack
    
    the code of the file until where you have to start completion: # This file is distributed under New Relic's license terms.
# See
    
      def call(env)
        Thread.new do
          100.times do
            env.each do |k, v|
              # allow main thread to run while we're still in the middle of
              # iterating
              Thread.pass
            end
          end
        end

        # Give the thread we just spawned a chance to start-up
        Thread.pass

        [200, {}, ['cool story']]
      end