SiblingSubgraph generates a HUGR with two output nodes

[CalMacCQ]

I've done a simple BFS to construct a subgraph from an input circuit and a starting node.

the input circuit HUGR looks like this. Load in this json file -> goto.json

After extracting the subgraph the HUGR we see that it has multiple output nodes which seems strange. See the extra ouput node hanging by itself on the right of the diagram below.

Sorry I don't know of a short way to reproduce this issue so I'll just share the code I hacked together.

use tket2::{
    self,
    hugr::{hugr::views::SiblingSubgraph, ops::NamedOp, HugrView, Node},
};

fn subgraph_from_source_node(circuit: &tket2::Circuit, source_node: Node) -> SiblingSubgraph {
    // Traverse the Circuit with BFS to find the subcircuit
    // in the causal cone of the source node

    let mut visited_nodes: Vec<Node> = Vec::new();
    let mut nodes_to_visit = vec![source_node];

    visited_nodes.push(source_node);

    while nodes_to_visit.len() > 0 {
        let current_node = nodes_to_visit.pop().unwrap();
        for neighbour in circuit.hugr().output_neighbours(current_node) {
            if !visited_nodes.contains(&neighbour) {
                visited_nodes.push(neighbour);
                nodes_to_visit.push(neighbour);
            }
        }
    }
    SiblingSubgraph::try_from_nodes(visited_nodes, circuit.hugr()).unwrap()
}

fn main() {
    let circ: tket2::Circuit = tket2::serialize::load_tk1_json_file("goto.json").unwrap();

    let pauli_z_cmds: Vec<_> = circ
        .commands()
        .filter(|cmd| cmd.optype().name() == "tket2.quantum.Z")
        .collect();

    let first_pauli = &pauli_z_cmds[0];

    let sibgraph = subgraph_from_source_node(&circ, first_pauli.node());

    let extracted = sibgraph.extract_subgraph(circ.hugr(), "");

    let subcircuit = tket2::Circuit::try_new(&extracted, extracted.root()).unwrap();

    println!("{}", subcircuit.mermaid_string());
}

[CalMacCQ]

More of a HUGR issue rather than tket2. I think it should be moved to that repository.