code: def _check_signatures(lines, **kwargs):
      
      trusted = kwargs.get("trusted", ())
      signatures = tuple(kwargs.get("signatures", ()))
      alt_signatures = tuple(kwargs.get("alt_signatures", ()))
      min_reviewers = kwargs.get("min_reviewers", 3)
  
      matching = []
      errors = []
      signatures += alt_signatures
  
      test_signatures = re.compile("^({0})".format("|".join(signatures)))
      test_alt_signatures = re.compile("^({0})".format("|".join(alt_signatures)))
      for i, line in lines:
          if signatures and test_signatures.search(line):
              if line.endswith("."):
                  errors.append(("M191", i))
              if not alt_signatures or not test_alt_signatures.search(line):
                  matching.append(line)
          else:
              errors.append(("M102", i))
  
      if not matching:
          errors.append(("M101", 1))
          errors.append(("M100", 1))
      elif len(matching) < min_reviewers:
          pattern = re.compile('|'.join(map(lambda x: '<' + re.escape(x) + '>',
                                            trusted)))
          trusted_matching = list(filter(None, map(pattern.search, matching)))
          if len(trusted_matching) == 0:
              errors.append(("M100", 1))
  
      return errors
docs: Check that the signatures are valid.
  
      There should be at least three signatures. If not, one of them should be a
      trusted developer/reviewer.
  
      Formatting supported being: [signature] full name <email@address>
  
      :param lines: lines (lineno, content) to verify.
      :type lines: list
      :param signatures: list of supported signature
      :type signatures: list
      :param alt_signatures: list of alternative signatures, not counted
      :type alt_signatures: list
      :param trusted: list of trusted reviewers, the e-mail address.
      :type trusted: list
      :param min_reviewers: minimal number of reviewers needed. (Default 3)
      :type min_reviewers: int
      :return: errors as in (code, line number, *args)
      :rtype: list