code: def count_citation_years(graph: BELGraph) -> typing.Counter[int]:
      
      result = defaultdict(set)
  
      for _, _, data in graph.edges(data=True):
          if CITATION not in data or CITATION_DATE not in data[CITATION]:
              continue
  
          try:
              dt = _ensure_datetime(data[CITATION][CITATION_DATE])
              result[dt.year].add((data[CITATION][CITATION_TYPE], data[CITATION][CITATION_REFERENCE]))
          except Exception:
              continue
  
      return count_dict_values(result)
docs: Count the number of citations from each year.