code: def clean(self):
          ''' Either staffCategory or staffMember must be filled in, but not both. '''
          if not self.staffCategory and not self.staffMember:
              raise ValidationError(_('Either staff category or staff member must be specified.'))
          if self.staffCategory and self.staffMember:
             raise ValidationError(_('Specify either a staff category or a staff member, not both.')f clean(self):
          ''' Either staffCategory or staffMember must be filled in, but not both. '''
          if not self.staffCategory and not self.staffMember:
              raise ValidationError(_('Either staff category or staff member must be specified.'))
          if self.staffCategory and self.staffMember:
             raise ValidationError(_('Specify either a staff category or a staff member, not both.'))
docs: Either staffCategory or staffMember must be filled in, but not both.