code: def load_cdx_for_dupe(self, url, timestamp, digest, cdx_loader):
          
          if not cdx_loader:
              return iter([])
  
          filters = []
  
          filters.append('!mime:warc/revisit')
  
          if digest and digest != '-':
              filters.append('digest:' + digest)
  
          params = dict(url=url,
                        closest=timestamp,
                        filter=filters)
  
          return cdx_loader(params)
docs: If a cdx_server is available, return response from server,
          otherwise empty list