问Graphviz嵌套子图方向

EN

digraph G {
   newrank=true
   splines=false  // A->C edge gets wacky without this
   
   node [shape=Mrecord]
   // Mrecords produce this Warning:
   //   flat edge between adjacent nodes one of which has a record shape - replace records with HTML-like labels
   // but Mrecords still seem to work, so maybe ignore warning ??

   // hoped that ordering or weight or group attributes would
   // position C and G as desired, but nope
   // instead, clusters and constraint attribute worked, why?

    A [group=T label="{Measure|4/4}"]
    B [group=T]
    A -> B
    {
      rank=same C F E D G   // declare right-to-left ??
    }
    subgraph clusterCDEFG {
        graph [style=rounded]
    // within a rank, layout tends to be right-to-left
    // so, declare right-to-left ??
    // why do these clusters help position C & G ???    
       subgraph clusterG {  peripheries=0
       G 
        }
        subgraph clusterDEF {
      // declare right-to-left
      F
          E [group=T]
      D 
      edge [style=invis]
       //   D -> E -> F
        }
    // why do these clusters help position C & G ???
        subgraph clusterC { peripheries=0
       C
        }
    }

    A -> C  [constraint=false]  // why does this impact position within rank ??
    A -> G  [constraint=false]  // why does this impact position within rank ??
    B -> { F E D } // declare right-to-left ??
    edge [style=invis]
//    C -> D  // Mrecord shape has problems
//    F -> G  // Mrecord shape has problems
}