gotrees.fs

let rec bfs2 (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 -> 
        bfs2 fanout tree item 
        |> Seq.append acc) single s

the above simpler code is valid if the map is guaranteed to be a tree without cycles,
otherwise, for a more robust but still functional implementation,look at https://gist.github.com/giuliohome/c9d4e4104e618d3cc94d2d09436c0952