<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _ItemsToUrns(self, items):
 """Converts collection items to aff4 urns suitable for downloading."""

    for item in items:
      try:
        yield flow_export.CollectionItemToAff4Path(item, self.client_id)
      except flow_export.ItemNotExportableError:
        pass