<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_from_sources(self, index, doc_type, document_id):
 """Get source stored locally"""

        return self.sources.get(index, {}).get(doc_type, {}).get(document_id, {})