code: def is_subdomain_record(rec):
      
      txt_entry = rec['txt']
      if not isinstance(txt_entry, list):
          return False
  
      has_parts_entry = False
      has_pk_entry = False
      has_seqn_entry = False
      for entry in txt_entry:
          if entry.startswith(SUBDOMAIN_ZF_PARTS + "="):
              has_parts_entry = True
          if entry.startswith(SUBDOMAIN_PUBKEY + "="):
              has_pk_entry = True
          if entry.startswith(SUBDOMAIN_N + "="):
              has_seqn_entry = True
  
      return (has_parts_entry and has_pk_entry and has_seqn_entry)
docs: Does a given parsed zone file TXT record (@rec) encode a subdomain?
      Return True if so
      Return False if not