docs: def get_convert_dict(fmt):
      """Retrieve parse definition from the format string `fmt`."""
      convdef = {}
      for literal_text, field_name, format_spec, conversion in formatter.parse(fmt):
          if field_name is None:
              continue
          # XXX: Do I need to include 'conversion'?
          convdef[field_name] = format_spec
      return convdef