docs: def update_asset_content(self, asset_content_form):
          """Updates an existing asset content.
  
          arg:    asset_content_form (osid.repository.AssetContentForm):
                  the form containing the elements to be updated
          raise:  IllegalState - ``asset_content_form`` already used in an
                  update transaction
          raise:  InvalidArgument - the form contains an invalid value
          raise:  NullArgument - ``asset_form`` is ``null``
          raise:  OperationFailed - unable to complete request
          raise:  PermissionDenied - authorization failure
          raise:  Unsupported - ``asset_content_form`` did not originate
                  from ``get_asset_content_form_for_update()``
          *compliance: mandatory -- This method must be implemented.*
  
          """
          # Implemented from template for
          # osid.repository.AssetAdminSession.update_asset_content_template
          from dlkit.abstract_osid.repository.objects import AssetContentForm as ABCAssetContentForm
          collection = JSONClientValidated('repository',
                                           collection='Asset',
                                           runtime=self._runtime)
          if not isinstance(asset_content_form, ABCAssetContentForm):
              raise errors.InvalidArgument('argument type is not an AssetContentForm')
          if not asset_content_form.is_for_update():
              raise errors.InvalidArgument('the AssetContentForm is for update only, not create')
          try:
              if self._forms[asset_content_form.get_id().get_identifier()] == UPDATED:
                  raise errors.IllegalState('asset_content_form already used in an update transaction')
          except KeyError:
              raise errors.Unsupported('asset_content_form did not originate from this session')
          if not asset_content_form.is_valid():
              raise errors.InvalidArgument('one or more of the form elements is invalid')
          asset_id = Id(asset_content_form._my_map['assetId']).get_identifier()
          asset = collection.find_one(
              {'$and': [{'_id': ObjectId(asset_id)},
                        {'assigned' + self._catalog_name + 'Ids': {'$in': [str(self._catalog_id)]}}]})
          index = 0
          found = False
          for i in asset['assetContents']:
              if i['_id'] == ObjectId(asset_content_form._my_map['_id']):
                  asset['assetContents'].pop(index)
                  asset['assetContents'].insert(index, asset_content_form._my_map)
                  found = True
                  break
              index += 1
          if not found:
              raise errors.NotFound()
          try:
              collection.save(asset)
          except:  # what exceptions does mongodb save raise?
              raise errors.OperationFailed()
          self._forms[asset_content_form.get_id().get_identifier()] = UPDATED
          # Note: this is out of spec. The OSIDs don't require an object to be returned:
          from .objects import AssetContent
  
          return AssetContent(
              osid_object_map=asset_content_form._my_map,
              runtime=self._runtime,
              proxy=self._proxy)