docs: def _parse_canonical_double(doc):
      """Decode a JSON double to python float."""
      d_str = doc['$numberDouble']
      if len(doc) != 1:
          raise TypeError('Bad $numberDouble, extra field(s): %s' % (doc,))
      if not isinstance(d_str, string_type):
          raise TypeError('$numberDouble must be string: %s' % (doc,))
      return float(d_str)