# File lib/rack/runtime.rb, line 16
    def call(env)
      start_time = clock_time
      status, headers, body = @app.call(env)
      request_time = clock_time - start_time

      if !headers.has_key?(@header_name)
        headers[@header_name] = FORMAT_STRING % request_time
      end

      [status, headers, body]
    end