code: def concatenate_and_rewrite(self, paths, output_filename, variant=None):
          
          stylesheets = []
          for path in paths:
              def reconstruct(match):
                  quote = match.group(1) or ''
                  asset_path = match.group(2)
                  if NON_REWRITABLE_URL.match(asset_path):
                      return "url(%s%s%s)" % (quote, asset_path, quote)
                  asset_url = self.construct_asset_path(asset_path, path,
                                                        output_filename, variant)
                  return "url(%s)" % asset_url
              content = self.read_text(path)
              # content needs to be unicode to avoid explosions with non-ascii chars
              content = re.sub(URL_DETECTOR, reconstruct, content)
              stylesheets.append(content)
          return '\n'.join(stylesheets)
docs: Concatenate together files and rewrite urls