code: def enrich_pubmed_citations(graph: BELGraph, manager: Manager) -> Set[str]:
      
      pmids = get_pubmed_identifiers(graph)
      pmid_data, errors = get_citations_by_pmids(manager=manager, pmids=pmids)
  
      for u, v, k in filter_edges(graph, has_pubmed):
          pmid = graph[u][v][k][CITATION][CITATION_REFERENCE].strip()
  
          if pmid not in pmid_data:
              log.warning('Missing data for PubMed identifier: %s', pmid)
              errors.add(pmid)
              continue
  
          graph[u][v][k][CITATION].update(pmid_data[pmid])
  
      return errors
docs: Overwrite all PubMed citations with values from NCBI's eUtils lookup service.
  
      :return: A set of PMIDs for which the eUtils service crashed