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