code: def track_parallel(items, sub_type):
      
      out = []
      for i, args in enumerate(items):
          item_i, item = _get_provitem_from_args(args)
          if item:
              sub_entity = "%s.%s.%s" % (item["provenance"]["entity"], sub_type, i)
              item["provenance"]["entity"] = sub_entity
              args = list(args)
              args[item_i] = item
          out.append(args)
      # TODO: store mapping of entity to sub identifiers
      return out
docs: Create entity identifiers to trace the given items in sub-commands.
  
      Helps handle nesting in parallel program execution:
  
      run id => sub-section id => parallel ids