code: def _srels_for(phys_reader, source_uri):
          
          rels_xml = phys_reader.rels_xml_for(source_uri)
          return _SerializedRelationshipCollection.load_from_xml(
              source_uri.baseURI, rels_xml)
docs: Return |_SerializedRelationshipCollection| instance populated with
          relationships for source identified by *source_uri*.