docs: def get_backup_blocks(cls, impl, working_dir):
          """
          Get the set of block IDs that were backed up
          """
          ret = []
          backup_dir = config.get_backups_directory(impl, working_dir)
          if not os.path.exists(backup_dir):
              return []
  
          for name in os.listdir( backup_dir ):
              if ".bak." not in name:
                  continue 
  
              suffix = name.split(".bak.")[-1]
              try:
                  block_id = int(suffix)
              except:
                  continue 
  
              # must exist...
              backup_paths = cls.get_backup_paths(block_id, impl, working_dir)
              for p in backup_paths:
                  if not os.path.exists(p):
                      # doesn't exist
                      block_id = None
                      continue
  
              if block_id is not None:
                  # have backup at this block 
                  ret.append(block_id)
  
          return ret