docs: def parse_serialdiff(sd_dict):
    "helper for translate_check"
    if isinstance(sd_dict,list):
      if len(sd_dict)!=2 or sd_dict[0]!='checkstale': raise NotImplementedError(sd_dict[0],len(sd_dict))
      return CheckStale(sd_dict[1])
    if isinstance(sd_dict['deltas'],list): # i.e. for VHString the whole deltas field is a single string
      # warning below: Delta.text will be a list sometimes. always?
      sd_dict['deltas']=[diff.Delta(d['slice']['a'],d['slice']['b'],d['replace']) for d in sd_dict['deltas']]
    return SerialDiff(**sd_dict)