if $0 ==__FILE__
  [nil].cycle.inject File.dirname( __FILE__ ) do | dir , |
    Dir.chdir dir
    file = "./main.rb"
    break(require file) if File.exist? file
    if dir.size < 4 then (p"require_err #{__FILE__}") ; exit end
    File.dirname( dir )
  end
end

Line 2: undefined method `cycle' for [nil]:Array (NoMethodError)