more robust check

safetraversal.fs

let rec bfs2robust (fanout: Map<'node, 'node seq> -> 'node -> 'node seq) (tree: Map<'node, 'node seq>) (node: 'node) : 'node seq = 
    let single = seq [node]
    match fanout tree node with
    | e when e = Seq.empty -> single
    | s -> Seq.fold (fun acc item -> 
        bfs2robust fanout (tree |> Map.remove node) item 
        |> Seq.append acc) single s |> Seq.distinct

for the simple case, if the map is guaranteed to be a tree without cycles, see https://gist.github.com/giuliohome/007d25d53d939966f4271acca6d73862