[Question] How to define this DAG with "_parallel"?

[dshintani404]

Now I consider the following DAG. (2, 3) and (4, 5) are executed in parallel, respectively.

As far as I thought, this DAG cannot be represented in a format of digdag. If it is modified as following, it can.

"+workflow":
  "+1": {}
  "+group":
    _parallel: 'true'
   "+3": {}
   "+group":
      "+2": {}
      "+group":
        _parallel: 'true'
        "+4": {}
        "+5": {}    
  "+6": {}

Is there any workarounds to define the first DAG?