<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def occurrence(self, file_name=None, path=None, date=None):
 """Add a file Occurrence. Args: file_name (str, optional):
 The file name for this occurrence. path (str, optional):
 The file path for this occurrence. date (str, optional):
 The datetime expression for this occurrence. Returns: obj: An instance of Occurrence. """

        if self._indicator_data.get('type') != 'File':
            # Indicator object has no logger to output warning
            return None

        occurrence_obj = FileOccurrence(file_name, path, date)
        self._occurrences.append(occurrence_obj)
        return occurrence_obj