docs: def header_body_from_content(content):
      """\
      Tries to extract the header and the message from the cable content.
  
      The header is something like
  
          UNCLASSIFIED ...
          SUBJECT ...
          REF ...
  
      while the message begins usually with a summary
  
          1. SUMMARY ...
          ...
          10. ...
      
      Returns (header, msg) or (None, None) if the header/message cannot be 
      detected.
      
      `content`
          The "content" part of a cable.
      """
      m = _CLASSIFIED_BY_PATTERN.search(content)
      idx = m and m.end() or 0
      m = _SUMMARY_PATTERN.search(content)
      summary_idx = m and m.start() or None
      m = _FIRST_PARAGRAPH_PATTERN.search(content)
      para_idx = m and m.start() or None
      if summary_idx and para_idx:
          idx = max(idx, min(summary_idx, para_idx))
      elif summary_idx:
          idx = max(summary_idx, idx)
      elif para_idx:
          idx = max(para_idx, idx)
      if idx > 0:
          return content[:idx], content[idx:]
      return None, None