Graphviz: How to place nodes only in lower semi-circle using circo layout?

Question

In the attached figure, the nodes are arranged in a circle around the node. Is there a (possibly generic) way to arrange the nodes only in the lower semi-circle, without having to provide fixed coordinates for the nodes?

Edit: Would like to achieve something like shown in the image attached below. As one can see - all the nodes are arranged in the lower semi-circular region (this figure was made using CMap Tools).

The code is trivial, but pasting it anyway.

digraph semicircle {

    rankdir="TD"
    graph [nodesep="0.1", ranksep="0.3", center=true]
    mindist="0.4"

    S [label="Root", style="filled", fillcolor="greenyellow", shape="box"]

    subgraph cluster1 {
             rank="same"
             A; B; C; D; 
             S -> {A, B, C, D};
    }    }

Answer 1

Try this:

digraph semicircle {

    rankdir="TD"
    graph [nodesep="0.1", ranksep="0.3", center=true, root=S]
    mindist="0.4"

    S [label="Root", style="filled", fillcolor="greenyellow", shape="box"]

    subgraph cluster1 {
             rank="same"
             A
             z1[style=invis label=""]
             z2[style=invis label=""]
             B; C; D;

             S -> A
             S -> z1,z2 [style=invis]
             S -> { B, C, D};
    }   
 }

Answer 2

using dot/circo : graphviz version 2.40.1 I noted that circo placed nodes counter-clockwise, starting at 3 o'clock. I added enough invisible nodes to fill the 2 through 10 o'clock positions. To make the inter-nodal distances even more uniform I added:

 node [shape=square style=rounded]

The result I got is this: