docs: def add_line(self, header_line):
          """Add header line, updating any necessary support indices
  
          :return: ``False`` on conflicting line and ``True`` otherwise
          """
          self.lines.append(header_line)
          self._indices.setdefault(header_line.key, OrderedDict())
          if not hasattr(header_line, "mapping"):
              return False  # no registration required
          if self.has_header_line(header_line.key, header_line.mapping["ID"]):
              warnings.warn(
                  (
                      "Detected duplicate header line with type {} and ID {}. "
                      "Ignoring this and subsequent one"
                  ).format(header_line.key, header_line.mapping["ID"]),
                  DuplicateHeaderLineWarning,
              )
              return False
          else:
              self._indices[header_line.key][header_line.mapping["ID"]] = header_line
              return True