How to use the bitstring.Bits._readhex function in bitstring

overlay_descriptor = ElementDescriptor(descriptor.code, descriptor.length, descriptor.scale, ref_value, descriptor.significance, descriptor.unit)
                    descriptor_overlay[descriptor.code] = overlay_descriptor
                    continue

                op_aaf = operators.get(OpCode.ADD_ASSOCIATED_FIELD, None)
                if op_aaf is not None and descriptor.code != fxy2int("031021"):
                    # Don't apply to ASSOCIATED FIELD SIGNIFICANCE
                    # Use dummy descriptor 999999 for associated field, like Geo::BUFR and libbufr
                    dummy_descriptors = iter([ElementDescriptor(fxy2int("999999"), op_aaf.bits(), 0, 0, "ASSOCIATED FIELD", "NUMERIC")])
                    vals = decode_compressed(bits, dummy_descriptors, n_subsets, {}, {})
                    for i,ss in enumerate(vals):
                        subsets[i].extend(ss)

                read_length = _calculate_read_length(descriptor, operators)
                if descriptor.unit == 'CCITTIA5':
                    ref_value = Bits._readhex(bits, read_length, bits.pos)
                else:
                    ref_value = Bits._readuint(bits, read_length, bits.pos)
                bits.pos += read_length

                n_bits = Bits._readuint(bits, 6, bits.pos)
                bits.pos += 6
                
                for i in range(n_subsets):
                    if descriptor.unit == 'CCITTIA5':
                        n_chars = n_bits
                        if n_chars:
                            raw_value = Bits._readhex(bits, n_chars*8, bits.pos)
                            bits.pos += n_chars*8
                            value = _decode_raw_value(raw_value, descriptor, operators)
                        else:
                            value = _decode_raw_value(ref_value, descriptor, operators)

read_length = _calculate_read_length(descriptor, operators)
                if descriptor.unit == 'CCITTIA5':
                    ref_value = Bits._readhex(bits, read_length, bits.pos)
                else:
                    ref_value = Bits._readuint(bits, read_length, bits.pos)
                bits.pos += read_length

                n_bits = Bits._readuint(bits, 6, bits.pos)
                bits.pos += 6
                
                for i in range(n_subsets):
                    if descriptor.unit == 'CCITTIA5':
                        n_chars = n_bits
                        if n_chars:
                            raw_value = Bits._readhex(bits, n_chars*8, bits.pos)
                            bits.pos += n_chars*8
                            value = _decode_raw_value(raw_value, descriptor, operators)
                        else:
                            value = _decode_raw_value(ref_value, descriptor, operators)
                    else:
                        if n_bits:
                            increment = Bits._readuint(bits, n_bits, bits.pos)
                            bits.pos += n_bits
                            if increment ^ ((1 << n_bits)-1) == 0: # Missing value, all-ones
                                value = _decode_raw_value((1 << descriptor.length)-1, descriptor, operators)
                            else:
                                value = _decode_raw_value(ref_value + increment, descriptor, operators)
                        else:
                            value = _decode_raw_value(ref_value, descriptor, operators)
                    subsets[i].append(value)
            elif isinstance(descriptor, ReplicationDescriptor):

overlay_descriptor = ElementDescriptor(descriptor.code, descriptor.length, descriptor.scale, ref_value, descriptor.significance, descriptor.unit)
                    descriptor_overlay[descriptor.code] = overlay_descriptor
                    continue
                
                op_aaf = operators.get(OpCode.ADD_ASSOCIATED_FIELD, None)
                if op_aaf is not None and descriptor.code != fxy2int("031021"):
                    # Don't apply to ASSOCIATED FIELD SIGNIFICANCE
                    associated_value = Bits._readuint(bits, op_aaf.bits(), bits.pos)
                    bits.pos += op_aaf.bits()
                    # Use dummy descriptor 999999 for associated field, like Geo::BUFR and libbufr
                    dummy_descriptor = ElementDescriptor(fxy2int("999999"), op_aaf.bits(), 0, 0, "ASSOCIATED FIELD", "NUMERIC")
                    values.append(BufrValue(associated_value, associated_value, dummy_descriptor))

                read_length = _calculate_read_length(descriptor, operators)
                if descriptor.unit == 'CCITTIA5':
                    raw_value = Bits._readhex(bits, read_length, bits.pos)
                else:
                    raw_value = Bits._readuint(bits, read_length, bits.pos)
                bits.pos += read_length
                values.append(_decode_raw_value(raw_value, descriptor, operators))
            elif isinstance(descriptor, ReplicationDescriptor):
                aggregation = []
                if descriptor.count:
                    bval = None
                    count = descriptor.count
                else:
                    bval = decode(bits, itertools.islice(descriptors, 1), {}, {})[0]
                    count = bval.value
                n_fields = descriptor.fields
                field_descriptors = list(itertools.islice(descriptors, n_fields))
                if bval is None or bval.descriptor.code in REPLICATION_DESCRIPTORS:
                    # Regular replication, X elements repeated Y or <element value=""> times in the file</element>

# Regular replication, X elements repeated Y or <element value=""> times in the file
                    for _ in range(count):
                        aggregation.append(decode(bits, iter(field_descriptors), operators, descriptor_overlay))
                elif bval.descriptor.code in REPETITION_DESCRIPTORS:
                    # Repeated replication, X elements present once in the file, output <element value=""> times
                    repeated_values = decode(bits, iter(field_descriptors), operators, descriptor_overlay)
                    for _ in range(count):
                        aggregation.append(repeated_values)
                else:
                    raise ValueError("Unexpected delayed replication element %s" %bval)
                values.append(aggregation)
            elif isinstance(descriptor, OperatorDescriptor):
                op = descriptor.operator
                if op.immediate:
                    if op.opcode == OpCode.SIGNIFY_CHARACTER:
                        raw_value = Bits._readhex(bits, op.bits(), bits.pos)
                        bits.pos += op.bits()
                        char_descriptor = ElementDescriptor(fxy2int(op.code), op.bits(), 0, 0, "CHARACTER INFORMATION", "CCITTIA5")
                        value = _decode_raw_value(raw_value, char_descriptor, {})
                        values.append(value)
                    elif op.opcode == OpCode.SIGNIFY_LOCAL_DESCRIPTOR:
                        base_descriptor = itertools.islice(descriptors, 1)[0]
                        mod_descriptor = ElementDescriptor(base_descriptor.code, op.bits(), base_descriptor.scale, base_descriptor.ref, base_descriptor.significance, base_descriptor.unit)
                        values.add(decode(bits, descriptors, {}, {})[0].value)
                        
                    else:
                        raise NotImplementedError("Unknown immediate operator: %s" % str(descriptor))
                else:
                    if op.neutral():
                        del operators[op.opcode]
                    else:
                        op.check_conflict(operators)</element></element>