# File lib/spruz/once.rb, line 7
    def only_once(lock_filename = nil, locking_constant = nil)
      lock_filename ||= $0
      locking_constant ||= LOCK_EX
      f = File.new(lock_filename, RDONLY)
      f.flock(locking_constant) and yield
    ensure
      if f
        f.flock LOCK_UN
        f.close
      end
    end