<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def track_parallel(items, sub_type):
 """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 """

    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