match tree, path with
| DiscriminationTree.Node (Some s, _), [] -> S.singleton (s, n)
| DiscriminationTree.Node (None, _), [] -> S.empty
match tree, path with
| DiscriminationTree.Node (Some s, _), [] -> S.singleton (s, n)
| DiscriminationTree.Node (None, _), [] -> S.empty