docs: def parse_signed_by(content, canonicalize=True):
      """\
      Returns a maybe empty iterable of signers of the cable.
  
      `content`
          The cable's content.
      `canonicalize`
          Indicates if the signers should be canonicalized (upper-case the
          string and remove typos) (enabled by default).
      """
      s = content[-300:]
      m = _SIGNER_PATTERN.search(s) or _SIGNER_PATTERN2.search(s)
      if not m:
          return []
      signers = m.group(1)
      tmp_signers = signers.upper()
      if tmp_signers in (u'CONFIDENTIAL', u'UNCLASSIFIED'):
          return []
      if canonicalize:
          tmp_signers = tuple(re.split(ur'\s+', tmp_signers.strip()))
          if tmp_signers == (u'KEEGANPAAL',): # 04TAIPEI3991
              signers = [u'KEEGAN', u'PAAL']
          elif tmp_signers == (u'JOHNSONKEANE',): # 05ASUNCION807
              signers = [u'JOHNSON', u'KEANE']
          elif tmp_signers == (u'STEWARTBALTIMORE',): # 06MUSCAT396
              signers = [u'STEWART', u'BALTIMORE']
          elif tmp_signers == (u'BIGUSBELL',): # 06KIRKUK112
              signers = [u'BIGUS', u'BELL']
          else:
              signers = tmp_signers
      else:
          signers = [signers]
      return [c14n.canonicalize_surname(s) for s in signers] if canonicalize else signers