I have a GraphMap from petgraph with a bunch of data building a parent child relationship between components.

I want to be able to be able to create a sub graph what is a collection of the child nodes and all of its dependants "downwards".

For example for A in the image i would just want a graph of A with C3 as its a dependant.

I understand there is a few algorithms in the petgraph crate but none of them can be used to create a subgraph? I couldn't find any good examples and im a bit stuck

```
use crate::petgraph::dot::Dot;
use petgraph::prelude::DiGraphMap;
use petgraph;
fn main() {
let mut g = DiGraphMap::new();
g.add_edge("A", "B", -1);
g.add_edge("A", "B2", -1);
g.add_edge("A", "B3", -1);
g.add_edge("B3", "C3", -1);
g.add_edge("cA", "cB", -1);
g.add_edge("cA", "cB2", -1);
g.add_edge("cA", "cB3", -1);
g.add_edge("cB3", "cC3", -1);
g.add_edge("cB3", "C3", -1);
println!("{}", format!("{}", Dot::new(&g)));
}
```

Output:

```
digraph {
0 [ label = "A" ]
1 [ label = "B" ]
2 [ label = "B2" ]
3 [ label = "B3" ]
4 [ label = "C3" ]
5 [ label = "cA" ]
6 [ label = "cB" ]
7 [ label = "cB2" ]
8 [ label = "cB3" ]
9 [ label = "cC3" ]
0 -> 1 [ label = "-1" ]
0 -> 2 [ label = "-1" ]
0 -> 3 [ label = "-1" ]
3 -> 4 [ label = "-1" ]
5 -> 6 [ label = "-1" ]
5 -> 7 [ label = "-1" ]
5 -> 8 [ label = "-1" ]
8 -> 9 [ label = "-1" ]
8 -> 4 [ label = "-1" ]
}
```