code: def check_crossrefs(self):
          
          broken = []
          for value in self.values():
              crossref = value.get('crossref')
              if crossref is not None:
                  if crossref not in self:
                      broken.append(value)
          if len(broken):
              raise exceptions.BrokenCrossReferences('One or more cross reference could not'
                      ' be resolved', broken)
docs: Checks all crossreferences found in the bibliography. If one can not
          be resolved from *this* Bibliography instance, a BrokenCrossReferences
          exception is raised.