docs: def match_prefix(prefix, line):
          """Check if the line starts with given prefix and
          return the position of the end of prefix.
          If the prefix is not matched, return -1.
          """
          m = re.match(prefix, line.expandtabs(4))
          if not m:
              if re.match(prefix, line.expandtabs(4).replace('\n', ' ' * 99 + '\n')):
                  return len(line) - 1
              return -1
          pos = m.end()
          if pos == 0:
              return 0
          for i in range(1, len(line) + 1):
              if len(line[:i].expandtabs(4)) >= pos:
                  return i