docs: def parse_subject(content, reference_id=None, clean=True):
      """\
      Parses and returns the subject of a cable. If the cable has no subject, an
      empty string is returned.
      
      `content`
          The cable's content.
      `reference_id`
          The (optional) reference id of the cable. Used for error msgs
      `clean`
          Indicates if classification prefixes like ``(S)`` should be removed from
          the subject (default: ``True``)
          * (TS) for Top Secret
          * (S) for Secret
          * (C) for Confidential
          * (U) for Unclassified
          * (SBU/N) for Sensitive But Unclassified/Noforn, and
          * (SBU) for Sensitive But Unclassified
          Source: 
          U.S. Department of State Foreign Affairs Handbook Volume 5 Handbook 1 — Correspondence Handbook
          5 FAH-1 H-210 -- HOW TO USE TELEGRAMS; page 2
          <http://www.state.gov/documents/organization/89319.pdf>
      """
      def to_unicodechar(match):
          return unichr(int(match.group(1)))
      m = _SUBJECT_MAX_PATTERN.search(content)
      max_idx = m.start() if m else _MAX_HEADER_IDX
      m = _SUBJECT_PATTERN.search(content, 0, max_idx)
      if not m:
          return u''
      res = m.group(1).strip()
      res = _NL_PATTERN.sub(u' ', res)
      res = _WS_PATTERN.sub(u' ', res)
      res = res.replace(u'US- ', u'US-')
      res = _HTML_ENTITIES_PATTERN.sub(to_unicodechar, res)
      if clean:
          res = _WS_PATTERN.sub(u' ', _SLASH_ESCAPE_PATTERN.sub(u'', _BRACES_PATTERN.sub(u'', res))).strip()
      return res