docs: def write_to(self, f, version = None):
          """
          Writes out the arc header to the file like object `f`. 
  
          If the version field is 1, it writes out an arc v1 header,
          otherwise (and this is default), it outputs a v2 header.
  
          """
          if not version:
              version = self.version
          if version == 1:
              header = "%(url)s %(ip_address)s %(date)s %(content_type)s %(length)s"
          elif version == 2:
              header = "%(url)s %(ip_address)s %(date)s %(content_type)s %(result_code)s %(checksum)s %(location)s %(offset)s %(filename)s %(length)s"
  
          header =  header%dict(url          = self['url'],
                                ip_address   = self['ip_address'],
                                date         = self['date'],
                                content_type = self['content_type'],
                                result_code  = self['result_code'],
                                checksum     = self['checksum'],
                                location     = self['location'],
                                offset       = self['offset'],
                                filename     = self['filename'],
                                length       = self['length'])
          f.write(header)