How to use the pyulog.core.ULog._FieldData function in pyulog

def _parse_nested_type(self, prefix_str, type_name, message_formats):
            # we flatten nested types
            message_format = message_formats[type_name]
            for (type_name_fmt, array_size, field_name) in message_format.fields:
                if type_name_fmt in ULog._UNPACK_TYPES:
                    if array_size > 1:
                        for i in range(array_size):
                            self.field_data.append(ULog._FieldData(
                                prefix_str+field_name+'['+str(i)+']', type_name_fmt))
                    else:
                        self.field_data.append(ULog._FieldData(
                            prefix_str+field_name, type_name_fmt))
                    if prefix_str+field_name == 'timestamp':
                        self.timestamp_idx = len(self.field_data) - 1
                else: # nested type
                    if array_size > 1:
                        for i in range(array_size):
                            self._parse_nested_type(prefix_str+field_name+'['+str(i)+'].',
                                                    type_name_fmt, message_formats)
                    else:
                        self._parse_nested_type(prefix_str+field_name+'.',
                                                type_name_fmt, message_formats)

def _parse_nested_type(self, prefix_str, type_name, message_formats):
            # we flatten nested types
            message_format = message_formats[type_name]
            for (type_name_fmt, array_size, field_name) in message_format.fields:
                if type_name_fmt in ULog._UNPACK_TYPES:
                    if array_size > 1:
                        for i in range(array_size):
                            self.field_data.append(ULog._FieldData(
                                prefix_str+field_name+'['+str(i)+']', type_name_fmt))
                    else:
                        self.field_data.append(ULog._FieldData(
                            prefix_str+field_name, type_name_fmt))
                    if prefix_str+field_name == 'timestamp':
                        self.timestamp_idx = len(self.field_data) - 1
                else: # nested type
                    if array_size > 1:
                        for i in range(array_size):
                            self._parse_nested_type(prefix_str+field_name+'['+str(i)+'].',
                                                    type_name_fmt, message_formats)
                    else:
                        self._parse_nested_type(prefix_str+field_name+'.',
                                                type_name_fmt, message_formats)