docs: def _check_rel(attrs, rel_whitelist, rel_blacklist):
          """ Check a link's relations against the whitelist or blacklist.
  
          First, this will reject based on blacklist.
  
          Next, if there is a whitelist, there must be at least one rel that matches.
          To explicitly allow links without a rel you can add None to the whitelist
          (e.g. ['in-reply-to',None])
          """
  
          rels = attrs.get('rel', [None])
  
          if rel_blacklist:
              # Never return True for a link whose rel appears in the blacklist
              for rel in rels:
                  if rel in rel_blacklist:
                      return False
  
          if rel_whitelist:
              # If there is a whitelist for rels, only return true for a rel that
              # appears in it
              for rel in rels:
                  if rel in rel_whitelist:
                      return True
              # If there is a whitelist and we don't match, then reject
              return False
  
          return True