docs: def sfilter(self, source):
          """Filter."""
  
          sources = []
          if source.text[:4].encode(source.encoding) != b'PK\x03\x04':
              sources.extend(self._filter(source.text, source.context, source.encoding))
          else:
              for content, filename, enc in self.get_content(io.BytesIO(source.text.encode(source.encoding))):
                  sources.extend(self._filter(content, source.context, enc))
          return sources