<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _csv_header(self):
 """ Extract the expected CSV header from the exposure metadata """

        fields = ['id', 'number', 'taxonomy', 'lon', 'lat']
        for name in self.cost_types['name']:
            fields.append(name)
        if 'per_area' in self.cost_types['type']:
            fields.append('area')
        if self.occupancy_periods:
            fields.extend(self.occupancy_periods.split())
        fields.extend(self.tagcol.tagnames)
        return set(fields)