code: def references(self):
          
          refs = self.items.find('bibrecord/tail/bibliography', ns)
          if refs is not None:
              eids = [r.find("ref-info/refd-itemidlist/itemid", ns).text for r
                      in refs.findall("reference", ns)]
              return ["2-s2.0-" + eid for eid in eids]
          else:
              return None
docs: Return EIDs of references of an article.
          Note: Requires the FULL view of the article.