    # File lib/rfilter/keyed_mailbox.rb, line 87
        def expire(age)
          cutoff = Time.now - (60 * 60 * 24) * age
          [ File.join(@path, 'new', '*'),
            File.join(@path, 'cur', '*'),
            File.join(@path, '.index', '*')].each { |glob|
            Dir[glob].each { |file|
              File::delete(file) if File::mtime(file) <= cutoff
            }
          }
        end